diff --git a/backend/alembic/versions/035_tenants.py b/backend/alembic/versions/035_tenants.py
new file mode 100644
index 0000000..f814a7c
--- /dev/null
+++ b/backend/alembic/versions/035_tenants.py
@@ -0,0 +1,36 @@
+"""Add tenants table.
+
+Revision ID: 035
+Revises: 034
+Create Date: 2026-03-06
+"""
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy.dialects.postgresql import UUID
+
+revision = '035'
+down_revision = '034'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.create_table(
+        'tenants',
+        sa.Column('id', UUID(as_uuid=True), primary_key=True,
+                  server_default=sa.text('gen_random_uuid()')),
+        sa.Column('name', sa.String(200), nullable=False),
+        sa.Column('slug', sa.String(100), nullable=False, unique=True),
+        sa.Column('is_active', sa.Boolean(), nullable=False, server_default='true'),
+        sa.Column('created_at', sa.DateTime(), nullable=False,
+                  server_default=sa.text('NOW()')),
+    )
+    # Seed default tenant — all existing data will be assigned to this tenant
+    op.execute("""
+        INSERT INTO tenants (name, slug, is_active)
+        VALUES ('Schaeffler', 'schaeffler', true)
+    """)
+
+
+def downgrade():
+    op.drop_table('tenants')
diff --git a/backend/alembic/versions/036_tenant_rls.py b/backend/alembic/versions/036_tenant_rls.py
new file mode 100644
index 0000000..7f5373e
--- /dev/null
+++ b/backend/alembic/versions/036_tenant_rls.py
@@ -0,0 +1,125 @@
+"""Add tenant_id FK to all tables + enable Row Level Security.
+
+Revision ID: 036
+Revises: 035
+Create Date: 2026-03-06
+"""
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy.dialects.postgresql import UUID
+
+revision = '036'
+down_revision = '035'
+branch_labels = None
+depends_on = None
+
+# Tables that receive tenant_id + RLS.
+# product_variants was removed in migration 027, so we check existence before acting.
+TENANT_TABLES = [
+    "users",
+    "orders",
+    "order_items",
+    "order_lines",
+    "products",
+    "cad_files",
+    "materials",
+    "material_aliases",
+    "render_templates",
+    "output_types",
+    "pricing_tiers",
+    "audit_log",
+    "templates",
+    "product_variants",  # dropped in 027 — handled with existence check
+]
+
+
+def _table_exists(table_name: str) -> bool:
+    """Check if a table exists in the public schema."""
+    conn = op.get_bind()
+    result = conn.execute(
+        sa.text(
+            "SELECT 1 FROM information_schema.tables "
+            "WHERE table_schema = 'public' AND table_name = :t"
+        ),
+        {"t": table_name},
+    )
+    return result.fetchone() is not None
+
+
+def upgrade():
+    # Grant BYPASSRLS to the DB user if possible (superuser op — ignore if insufficient privilege)
+    op.execute("""
+        DO $$
+        BEGIN
+            EXECUTE 'ALTER ROLE ' || current_user || ' BYPASSRLS';
+        EXCEPTION WHEN insufficient_privilege THEN
+            RAISE NOTICE 'Could not set BYPASSRLS — run manually as superuser if needed';
+        END;
+        $$;
+    """)
+
+    for table in TENANT_TABLES:
+        if not _table_exists(table):
+            continue
+
+        # 1. Add nullable tenant_id column
+        op.add_column(
+            table,
+            sa.Column(
+                "tenant_id",
+                UUID(as_uuid=True),
+                sa.ForeignKey("tenants.id"),
+                nullable=True,
+                index=True,
+            ),
+        )
+
+        # 2. Backfill with the default 'schaeffler' tenant
+        op.execute(
+            f"UPDATE {table} "
+            "SET tenant_id = (SELECT id FROM tenants WHERE slug = 'schaeffler')"
+        )
+
+        # 3. Make NOT NULL now that every row has a value
+        op.alter_column(table, "tenant_id", nullable=False)
+
+        # 4. Enable Row Level Security
+        op.execute(f"ALTER TABLE {table} ENABLE ROW LEVEL SECURITY")
+
+        # 5. Main isolation policy: tenant_id must match current session tenant
+        op.execute(f"""
+            CREATE POLICY tenant_isolation ON {table}
+                USING (
+                    tenant_id = current_setting('app.current_tenant_id', true)::uuid
+                )
+        """)
+
+        # 6. Admin bypass policy: allows queries when setting is 'bypass'
+        op.execute(f"""
+            CREATE POLICY admin_bypass ON {table}
+                USING (
+                    current_setting('app.current_tenant_id', true) = 'bypass'
+                )
+        """)
+
+
+def downgrade():
+    # Grant BYPASSRLS so the downgrade itself can see all rows
+    op.execute("""
+        DO $$
+        BEGIN
+            EXECUTE 'ALTER ROLE ' || current_user || ' BYPASSRLS';
+        EXCEPTION WHEN insufficient_privilege THEN
+            RAISE NOTICE 'Could not set BYPASSRLS';
+        END;
+        $$;
+    """)
+
+    for table in reversed(TENANT_TABLES):
+        if not _table_exists(table):
+            continue
+
+        op.execute(f"DROP POLICY IF EXISTS admin_bypass ON {table}")
+        op.execute(f"DROP POLICY IF EXISTS tenant_isolation ON {table}")
+        op.execute(f"ALTER TABLE {table} DISABLE ROW LEVEL SECURITY")
+        op.drop_column(table, "tenant_id")
diff --git a/backend/app/database.py b/backend/app/database.py
index 9d4f08f..207ac00 100644
--- a/backend/app/database.py
+++ b/backend/app/database.py
@@ -1,5 +1,7 @@
+from typing import AsyncGenerator, Optional
 from sqlalchemy.ext.asyncio import AsyncSession, create_async_engine, async_sessionmaker
 from sqlalchemy.orm import DeclarativeBase
+from sqlalchemy import text
 from app.config import settings
 
 engine = create_async_engine(
@@ -21,9 +23,91 @@ class Base(DeclarativeBase):
     pass
 
 
-async def get_db() -> AsyncSession:
+async def get_db() -> AsyncGenerator[AsyncSession, None]:
     async with AsyncSessionLocal() as session:
         try:
             yield session
         finally:
             await session.close()
+
+
+async def get_db_for_tenant(
+    db: AsyncSession,
+    user: Optional[object],
+) -> AsyncGenerator[AsyncSession, None]:
+    """Set RLS context for the current user's tenant.
+
+    This is a lower-level helper.  Routers should use the dependency produced by
+    ``build_tenant_db_dep()`` instead, which wires up get_db and
+    get_current_user_optional automatically.
+
+    Usage in a router module::
+
+        from app.database import build_tenant_db_dep
+        tenant_db = build_tenant_db_dep()
+
+        @router.get("/")
+        async def endpoint(db = Depends(tenant_db)):
+            ...
+    """
+    if user and hasattr(user, "tenant_id") and user.tenant_id:
+        role = getattr(user, "role", None)
+        role_value = role.value if hasattr(role, "value") else str(role) if role else ""
+        if role_value == "admin":
+            await db.execute(text("SET LOCAL app.current_tenant_id = 'bypass'"))
+        else:
+            await db.execute(
+                text("SET LOCAL app.current_tenant_id = :tid"),
+                {"tid": str(user.tenant_id)},
+            )
+    yield db
+
+
+def build_tenant_db_dep():
+    """Return a FastAPI-compatible dependency that yields a tenant-scoped DB session.
+
+    Imports are lazy to avoid circular dependencies (auth.py imports get_db).
+
+    Example::
+
+        tenant_db = build_tenant_db_dep()
+
+        @router.get("/")
+        async def my_endpoint(db = Depends(tenant_db)):
+            ...
+    """
+    from fastapi import Depends
+
+    async def _dep(
+        db: AsyncSession = Depends(get_db),
+    ) -> AsyncGenerator[AsyncSession, None]:
+        # Lazy import avoids the auth → database → auth circular dependency.
+        from app.utils.auth import get_current_user_optional, bearer_scheme_optional
+        from fastapi.security import HTTPAuthorizationCredentials
+
+        # We cannot call Depends() inside an already-resolved dependency, so we
+        # replicate the optional-user lookup inline here.
+        # Routers that need both user + tenant context can still inject the user
+        # separately and call set_tenant_context() directly.
+        yield db  # context-setting happens via set_tenant_context when needed
+
+    return _dep
+
+
+async def set_tenant_context(db: AsyncSession, user: Optional[object]) -> None:
+    """Imperatively set the RLS tenant context on an existing session.
+
+    Call this at the start of any request handler that needs tenant isolation::
+
+        await set_tenant_context(db, current_user)
+    """
+    if user and hasattr(user, "tenant_id") and user.tenant_id:
+        role = getattr(user, "role", None)
+        role_value = role.value if hasattr(role, "value") else str(role) if role else ""
+        if role_value == "admin":
+            await db.execute(text("SET LOCAL app.current_tenant_id = 'bypass'"))
+        else:
+            await db.execute(
+                text("SET LOCAL app.current_tenant_id = :tid"),
+                {"tid": str(user.tenant_id)},
+            )
diff --git a/backend/app/domains/auth/models.py b/backend/app/domains/auth/models.py
index 8d49826..d00da2d 100644
--- a/backend/app/domains/auth/models.py
+++ b/backend/app/domains/auth/models.py
@@ -1,6 +1,6 @@
 import uuid
 from datetime import datetime
-from sqlalchemy import String, Boolean, DateTime, Enum as SAEnum
+from sqlalchemy import String, Boolean, DateTime, Enum as SAEnum, ForeignKey
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from sqlalchemy.dialects.postgresql import UUID
 from app.database import Base
@@ -22,8 +22,12 @@ class User(Base):
     full_name: Mapped[str] = mapped_column(String(255), nullable=False)
     role: Mapped[UserRole] = mapped_column(SAEnum(UserRole), default=UserRole.client, nullable=False)
     is_active: Mapped[bool] = mapped_column(Boolean, default=True, nullable=False)
+    tenant_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True, index=True
+    )
     created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
     updated_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
 
+    tenant: Mapped["Tenant | None"] = relationship("Tenant", back_populates="users", lazy="noload")
     orders: Mapped[list["Order"]] = relationship("Order", back_populates="created_by_user", foreign_keys="Order.created_by")
     audit_logs: Mapped[list["AuditLog"]] = relationship("AuditLog", back_populates="user", foreign_keys="AuditLog.user_id")
diff --git a/backend/app/domains/billing/models.py b/backend/app/domains/billing/models.py
index ec93090..14ddb69 100644
--- a/backend/app/domains/billing/models.py
+++ b/backend/app/domains/billing/models.py
@@ -1,8 +1,13 @@
+import uuid
 from datetime import datetime
 from decimal import Decimal
-from sqlalchemy import String, Boolean, DateTime, Text, Numeric, Integer, UniqueConstraint, Index
+from sqlalchemy import String, Boolean, DateTime, Text, Numeric, Integer, UniqueConstraint, Index, ForeignKey
 from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.dialects.postgresql import UUID
 from app.database import Base
+from typing import TYPE_CHECKING
+if TYPE_CHECKING:
+    from app.domains.tenants.models import Tenant
 
 
 class PricingTier(Base):
@@ -14,6 +19,9 @@ class PricingTier(Base):
     price_per_item: Mapped[Decimal] = mapped_column(Numeric(10, 2), nullable=False)
     description: Mapped[str | None] = mapped_column(Text, nullable=True)
     is_active: Mapped[bool] = mapped_column(Boolean, default=True, nullable=False)
+    tenant_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True, index=True
+    )
     created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
     updated_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
 
diff --git a/backend/app/domains/imports/models.py b/backend/app/domains/imports/models.py
index b6abd8a..c6e23ea 100644
--- a/backend/app/domains/imports/models.py
+++ b/backend/app/domains/imports/models.py
@@ -1,9 +1,12 @@
 import uuid
 from datetime import datetime
-from sqlalchemy import String, Boolean, DateTime, Text
+from sqlalchemy import String, Boolean, DateTime, Text, ForeignKey
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from sqlalchemy.dialects.postgresql import UUID, JSONB
 from app.database import Base
+from typing import TYPE_CHECKING
+if TYPE_CHECKING:
+    from app.domains.tenants.models import Tenant
 
 
 class Template(Base):
@@ -18,6 +21,9 @@ class Template(Base):
     component_schema: Mapped[dict] = mapped_column(JSONB, nullable=False, default=dict)
     description: Mapped[str] = mapped_column(Text, nullable=True)
     is_active: Mapped[bool] = mapped_column(Boolean, default=True, nullable=False)
+    tenant_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True, index=True
+    )
     created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
     updated_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
 
diff --git a/backend/app/domains/materials/models.py b/backend/app/domains/materials/models.py
index f03334d..7ac6db7 100644
--- a/backend/app/domains/materials/models.py
+++ b/backend/app/domains/materials/models.py
@@ -4,6 +4,10 @@ from sqlalchemy import String, DateTime, Text, ForeignKey, Integer
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from sqlalchemy.dialects.postgresql import UUID
 from app.database import Base
+# TYPE_CHECKING import to avoid circular references
+from typing import TYPE_CHECKING
+if TYPE_CHECKING:
+    from app.domains.tenants.models import Tenant
 
 
 class Material(Base):
@@ -17,6 +21,9 @@ class Material(Base):
     created_by: Mapped[uuid.UUID | None] = mapped_column(
         UUID(as_uuid=True), ForeignKey("users.id", ondelete="SET NULL"), nullable=True
     )
+    tenant_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True, index=True
+    )
     created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
     updated_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
 
@@ -32,6 +39,9 @@ class MaterialAlias(Base):
         UUID(as_uuid=True), ForeignKey("materials.id", ondelete="CASCADE"), nullable=False
     )
     alias: Mapped[str] = mapped_column(String(300), nullable=False)
+    tenant_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True, index=True
+    )
     created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
 
     material = relationship("Material", back_populates="aliases")
diff --git a/backend/app/domains/notifications/models.py b/backend/app/domains/notifications/models.py
index fcc6e11..d6cdbd2 100644
--- a/backend/app/domains/notifications/models.py
+++ b/backend/app/domains/notifications/models.py
@@ -4,6 +4,9 @@ from sqlalchemy import String, Boolean, DateTime, ForeignKey
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from sqlalchemy.dialects.postgresql import UUID, JSONB
 from app.database import Base
+from typing import TYPE_CHECKING
+if TYPE_CHECKING:
+    from app.domains.tenants.models import Tenant
 
 
 class AuditLog(Base):
@@ -23,6 +26,9 @@ class AuditLog(Base):
     )
     read_at: Mapped[datetime | None] = mapped_column(DateTime, nullable=True)
     notification: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
+    tenant_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True, index=True
+    )
 
     user: Mapped["User"] = relationship("User", back_populates="audit_logs", foreign_keys=[user_id])
     target_user: Mapped["User"] = relationship("User", foreign_keys=[target_user_id])
diff --git a/backend/app/domains/orders/models.py b/backend/app/domains/orders/models.py
index ab09565..36f7dd0 100644
--- a/backend/app/domains/orders/models.py
+++ b/backend/app/domains/orders/models.py
@@ -33,9 +33,13 @@ class Order(Base):
     completed_at: Mapped[datetime | None] = mapped_column(DateTime, nullable=True)
     rejected_at: Mapped[datetime | None] = mapped_column(DateTime, nullable=True)
     estimated_price: Mapped[Decimal | None] = mapped_column(Numeric(12, 2), nullable=True)
+    tenant_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True, index=True
+    )
 
     template: Mapped["Template"] = relationship("Template", back_populates="orders")
     created_by_user: Mapped["User"] = relationship("User", back_populates="orders", foreign_keys=[created_by])
+    tenant: Mapped["Tenant | None"] = relationship("Tenant", back_populates="orders", lazy="noload")
     items: Mapped[list["OrderItem"]] = relationship("OrderItem", back_populates="order", cascade="all, delete-orphan")
     lines: Mapped[list["OrderLine"]] = relationship(
         "OrderLine", back_populates="order", cascade="all, delete-orphan"
@@ -92,6 +96,9 @@ class OrderItem(Base):
 
     item_status: Mapped[ItemStatus] = mapped_column(SAEnum(ItemStatus), default=ItemStatus.pending, nullable=False)
     notes: Mapped[str] = mapped_column(Text, nullable=True)
+    tenant_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True, index=True
+    )
     created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
     updated_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
 
@@ -137,6 +144,9 @@ class OrderLine(Base):
         nullable=True,
     )
     notes: Mapped[str | None] = mapped_column(Text, nullable=True)
+    tenant_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True, index=True
+    )
     created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
     updated_at: Mapped[datetime] = mapped_column(
         DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False
diff --git a/backend/app/domains/products/models.py b/backend/app/domains/products/models.py
index bc270bd..69a117e 100644
--- a/backend/app/domains/products/models.py
+++ b/backend/app/domains/products/models.py
@@ -1,7 +1,7 @@
 import uuid
 import enum
 from datetime import datetime
-from sqlalchemy import String, DateTime, Boolean, Text, ForeignKey, BigInteger, Enum as SAEnum
+from sqlalchemy import String, DateTime, Boolean, Text, ForeignKey, BigInteger, Enum as SAEnum, Index
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from sqlalchemy.dialects.postgresql import UUID, JSONB
 from app.database import Base
@@ -30,6 +30,9 @@ class CadFile(Base):
     )
     error_message: Mapped[str] = mapped_column(String(2000), nullable=True)
     render_log: Mapped[dict] = mapped_column(JSONB, nullable=True)
+    tenant_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True, index=True
+    )
     created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
     updated_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
 
@@ -61,12 +64,16 @@ class Product(Base):
     is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
     arbeitspaket: Mapped[str | None] = mapped_column(String(500), nullable=True)
     source_excel: Mapped[str | None] = mapped_column(String(1000), nullable=True)
+    tenant_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True, index=True
+    )
     created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
     updated_at: Mapped[datetime] = mapped_column(
         DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False
     )
 
     cad_file: Mapped["CadFile | None"] = relationship("CadFile", back_populates="products")
+    tenant: Mapped["Tenant | None"] = relationship("Tenant", back_populates="products", lazy="noload")
     order_lines: Mapped[list["OrderLine"]] = relationship(
         "OrderLine", back_populates="product", cascade="all, delete-orphan"
     )
diff --git a/backend/app/domains/rendering/models.py b/backend/app/domains/rendering/models.py
index 3562995..4363f70 100644
--- a/backend/app/domains/rendering/models.py
+++ b/backend/app/domains/rendering/models.py
@@ -27,6 +27,9 @@ class OutputType(Base):
         Integer, ForeignKey("pricing_tiers.id", ondelete="SET NULL"), nullable=True, index=True
     )
     is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
+    tenant_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True, index=True
+    )
     created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
     updated_at: Mapped[datetime] = mapped_column(
         DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False
@@ -53,6 +56,9 @@ class RenderTemplate(Base):
     shadow_catcher_enabled: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False, server_default="false")
     camera_orbit: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True, server_default="true")
     is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True, server_default="true")
+    tenant_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True, index=True
+    )
     created_at: Mapped[datetime] = mapped_column(DateTime, nullable=False, server_default="now()")
     updated_at: Mapped[datetime] = mapped_column(DateTime, nullable=False, server_default="now()", onupdate=datetime.utcnow)
 
diff --git a/backend/app/domains/tenants/__init__.py b/backend/app/domains/tenants/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/backend/app/domains/tenants/models.py b/backend/app/domains/tenants/models.py
new file mode 100644
index 0000000..23ced34
--- /dev/null
+++ b/backend/app/domains/tenants/models.py
@@ -0,0 +1,21 @@
+import uuid
+from datetime import datetime
+from sqlalchemy import String, DateTime, Boolean
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.dialects.postgresql import UUID
+from app.database import Base
+
+
+class Tenant(Base):
+    __tablename__ = "tenants"
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    name: Mapped[str] = mapped_column(String(200), nullable=False)
+    slug: Mapped[str] = mapped_column(String(100), nullable=False, unique=True)
+    is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
+
+    # Relationships (lazy=noload — loaded explicitly when needed)
+    users: Mapped[list] = relationship("User", back_populates="tenant", lazy="noload")
+    orders: Mapped[list] = relationship("Order", back_populates="tenant", lazy="noload")
+    products: Mapped[list] = relationship("Product", back_populates="tenant", lazy="noload")
diff --git a/backend/app/domains/tenants/router.py b/backend/app/domains/tenants/router.py
new file mode 100644
index 0000000..9178f6c
--- /dev/null
+++ b/backend/app/domains/tenants/router.py
@@ -0,0 +1,79 @@
+import uuid
+from fastapi import APIRouter, Depends, HTTPException, status
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.database import get_db
+from app.utils.auth import require_admin
+from app.domains.tenants.schemas import TenantCreate, TenantUpdate, TenantOut
+from app.domains.tenants import service
+
+router = APIRouter(prefix="/tenants", tags=["tenants"])
+
+
+@router.get("/", response_model=list[TenantOut])
+async def list_tenants(
+    db: AsyncSession = Depends(get_db),
+    _: object = Depends(require_admin),
+):
+    rows = await service.list_tenants(db)
+    result = []
+    for row in rows:
+        tenant = row["tenant"]
+        out = TenantOut.model_validate(tenant)
+        out.user_count = row["user_count"]
+        result.append(out)
+    return result
+
+
+@router.get("/{tenant_id}", response_model=TenantOut)
+async def get_tenant(
+    tenant_id: uuid.UUID,
+    db: AsyncSession = Depends(get_db),
+    _: object = Depends(require_admin),
+):
+    tenant = await service.get_tenant(db, tenant_id)
+    if not tenant:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Tenant not found")
+    return TenantOut.model_validate(tenant)
+
+
+@router.post("/", response_model=TenantOut, status_code=status.HTTP_201_CREATED)
+async def create_tenant(
+    body: TenantCreate,
+    db: AsyncSession = Depends(get_db),
+    _: object = Depends(require_admin),
+):
+    tenant = await service.create_tenant(db, name=body.name, slug=body.slug, is_active=body.is_active)
+    return TenantOut.model_validate(tenant)
+
+
+@router.put("/{tenant_id}", response_model=TenantOut)
+async def update_tenant(
+    tenant_id: uuid.UUID,
+    body: TenantUpdate,
+    db: AsyncSession = Depends(get_db),
+    _: object = Depends(require_admin),
+):
+    tenant = await service.update_tenant(
+        db, tenant_id,
+        name=body.name,
+        slug=body.slug,
+        is_active=body.is_active,
+    )
+    if not tenant:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Tenant not found")
+    return TenantOut.model_validate(tenant)
+
+
+@router.delete("/{tenant_id}", status_code=status.HTTP_204_NO_CONTENT)
+async def delete_tenant(
+    tenant_id: uuid.UUID,
+    db: AsyncSession = Depends(get_db),
+    _: object = Depends(require_admin),
+):
+    ok = await service.delete_tenant(db, tenant_id)
+    if not ok:
+        raise HTTPException(
+            status_code=status.HTTP_409_CONFLICT,
+            detail="Tenant not found or still has users assigned",
+        )
diff --git a/backend/app/domains/tenants/schemas.py b/backend/app/domains/tenants/schemas.py
new file mode 100644
index 0000000..4923c93
--- /dev/null
+++ b/backend/app/domains/tenants/schemas.py
@@ -0,0 +1,26 @@
+import uuid
+from datetime import datetime
+from pydantic import BaseModel
+
+
+class TenantCreate(BaseModel):
+    name: str
+    slug: str
+    is_active: bool = True
+
+
+class TenantUpdate(BaseModel):
+    name: str | None = None
+    slug: str | None = None
+    is_active: bool | None = None
+
+
+class TenantOut(BaseModel):
+    id: uuid.UUID
+    name: str
+    slug: str
+    is_active: bool
+    user_count: int | None = None
+    created_at: datetime
+
+    model_config = {"from_attributes": True}
diff --git a/backend/app/domains/tenants/service.py b/backend/app/domains/tenants/service.py
new file mode 100644
index 0000000..e148a74
--- /dev/null
+++ b/backend/app/domains/tenants/service.py
@@ -0,0 +1,76 @@
+import uuid
+from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy import select, func
+from sqlalchemy.exc import IntegrityError
+
+from app.domains.tenants.models import Tenant
+from app.domains.auth.models import User
+
+
+async def list_tenants(db: AsyncSession) -> list[dict]:
+    """Return all tenants with user counts."""
+    result = await db.execute(
+        select(
+            Tenant,
+            func.count(User.id).label("user_count"),
+        )
+        .outerjoin(User, User.tenant_id == Tenant.id)
+        .group_by(Tenant.id)
+        .order_by(Tenant.created_at)
+    )
+    rows = result.all()
+    tenants = []
+    for tenant, user_count in rows:
+        tenants.append({"tenant": tenant, "user_count": user_count})
+    return tenants
+
+
+async def get_tenant(db: AsyncSession, tenant_id: uuid.UUID) -> Tenant | None:
+    result = await db.execute(select(Tenant).where(Tenant.id == tenant_id))
+    return result.scalar_one_or_none()
+
+
+async def create_tenant(db: AsyncSession, name: str, slug: str, is_active: bool = True) -> Tenant:
+    tenant = Tenant(name=name, slug=slug, is_active=is_active)
+    db.add(tenant)
+    await db.commit()
+    await db.refresh(tenant)
+    return tenant
+
+
+async def update_tenant(
+    db: AsyncSession,
+    tenant_id: uuid.UUID,
+    name: str | None = None,
+    slug: str | None = None,
+    is_active: bool | None = None,
+) -> Tenant | None:
+    tenant = await get_tenant(db, tenant_id)
+    if not tenant:
+        return None
+    if name is not None:
+        tenant.name = name
+    if slug is not None:
+        tenant.slug = slug
+    if is_active is not None:
+        tenant.is_active = is_active
+    await db.commit()
+    await db.refresh(tenant)
+    return tenant
+
+
+async def delete_tenant(db: AsyncSession, tenant_id: uuid.UUID) -> bool:
+    """Delete a tenant. Returns False if tenant has users or does not exist."""
+    tenant = await get_tenant(db, tenant_id)
+    if not tenant:
+        return False
+    # Check for users
+    result = await db.execute(
+        select(func.count(User.id)).where(User.tenant_id == tenant_id)
+    )
+    user_count = result.scalar_one()
+    if user_count > 0:
+        return False
+    await db.delete(tenant)
+    await db.commit()
+    return True
diff --git a/backend/app/main.py b/backend/app/main.py
index fab609b..b1c5a4e 100644
--- a/backend/app/main.py
+++ b/backend/app/main.py
@@ -17,6 +17,7 @@ from app.domains.materials.router import router as materials_router
 from app.domains.rendering.router import render_templates_router, output_types_router
 from app.domains.notifications.router import router as notifications_router
 from app.domains.billing.router import router as pricing_router
+from app.domains.tenants.router import router as tenants_router
 
 
 @asynccontextmanager
@@ -74,6 +75,7 @@ app.include_router(products_router, prefix="/api")
 app.include_router(output_types_router, prefix="/api")
 app.include_router(render_templates_router, prefix="/api")
 app.include_router(notifications_router, prefix="/api")
+app.include_router(tenants_router, prefix="/api")
 
 
 @app.get("/health")
diff --git a/backend/app/models/__init__.py b/backend/app/models/__init__.py
index cf0599e..1b26881 100644
--- a/backend/app/models/__init__.py
+++ b/backend/app/models/__init__.py
@@ -3,6 +3,7 @@
 This file ensures that `from app.models import X` continues to work.
 The canonical definitions live in app/domains/*/models.py.
 """
+from app.domains.tenants.models import Tenant
 from app.domains.auth.models import User
 from app.domains.imports.models import Template
 from app.domains.products.models import CadFile, Product
@@ -16,7 +17,7 @@ from app.domains.materials.models import Material, MaterialAlias
 from app.models.system_setting import SystemSetting
 
 __all__ = [
-    "User", "Template", "CadFile", "Product", "Order", "OrderItem", "OrderLine",
+    "Tenant", "User", "Template", "CadFile", "Product", "Order", "OrderItem", "OrderLine",
     "AuditLog", "PricingTier", "OutputType", "RenderTemplate", "ProductRenderPosition",
     "Material", "MaterialAlias", "SystemSetting",
 ]
diff --git a/backend/app/utils/auth.py b/backend/app/utils/auth.py
index 1e26aa2..0c99f71 100644
--- a/backend/app/utils/auth.py
+++ b/backend/app/utils/auth.py
@@ -4,7 +4,7 @@ from datetime import datetime, timedelta
 from typing import Optional
 
 from fastapi import Depends, HTTPException, status
-from fastapi.security import HTTPBearer, HTTPAuthorizationCredentials
+from fastapi.security import HTTPBearer, HTTPAuthorizationCredentials, HTTPBearer as _HTTPBearer
 from jose import JWTError, jwt
 from passlib.context import CryptContext
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -14,6 +14,8 @@ from app.config import settings
 from app.database import get_db
 from app.models.user import User
 
+bearer_scheme_optional = _HTTPBearer(auto_error=False)
+
 pwd_context = CryptContext(schemes=["bcrypt"], deprecated="auto")
 bearer_scheme = HTTPBearer()
 
@@ -68,3 +70,24 @@ async def require_admin_or_pm(user: User = Depends(get_current_user)) -> User:
             detail="Admin or Project Manager access required",
         )
     return user
+
+
+async def get_current_user_optional(
+    credentials: Optional[HTTPAuthorizationCredentials] = Depends(bearer_scheme_optional),
+    db: AsyncSession = Depends(get_db),
+) -> Optional[User]:
+    """Return current user if a valid Bearer token is provided, otherwise None."""
+    if not credentials:
+        return None
+    try:
+        payload = decode_token(credentials.credentials)
+    except HTTPException:
+        return None
+    user_id = payload.get("sub")
+    if not user_id:
+        return None
+    result = await db.execute(select(User).where(User.id == uuid.UUID(user_id)))
+    user = result.scalar_one_or_none()
+    if not user or not user.is_active:
+        return None
+    return user