diff --git a/alembic/versions/z_add_unique_subdomain_domain_constraints.py b/alembic/versions/z_add_unique_subdomain_domain_constraints.py
new file mode 100644
index 00000000..33064bc1
--- /dev/null
+++ b/alembic/versions/z_add_unique_subdomain_domain_constraints.py
@@ -0,0 +1,33 @@
+"""add unique constraints for custom_subdomain and store domain per platform
+
+Revision ID: z_unique_subdomain_domain
+Revises: a44f4956cfb1
+Create Date: 2026-02-26
+"""
+from alembic import op
+
+revision = "z_unique_subdomain_domain"
+down_revision = ("a44f4956cfb1", "tenancy_003")
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # StorePlatform: same custom_subdomain cannot be claimed twice on the same platform
+    op.create_unique_constraint(
+        "uq_custom_subdomain_platform",
+        "store_platforms",
+        ["custom_subdomain", "platform_id"],
+    )
+
+    # StoreDomain: a store can have at most one custom domain per platform
+    op.create_unique_constraint(
+        "uq_store_domain_platform",
+        "store_domains",
+        ["store_id", "platform_id"],
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint("uq_store_domain_platform", "store_domains", type_="unique")
+    op.drop_constraint("uq_custom_subdomain_platform", "store_platforms", type_="unique")
diff --git a/app/modules/tenancy/models/store_domain.py b/app/modules/tenancy/models/store_domain.py
index 476c1677..49e5a78d 100644
--- a/app/modules/tenancy/models/store_domain.py
+++ b/app/modules/tenancy/models/store_domain.py
@@ -68,6 +68,8 @@ class StoreDomain(Base, TimestampMixin):
     # Constraints
     __table_args__ = (
         UniqueConstraint("store_id", "domain", name="uq_store_domain"),
+        # A store can have at most one custom domain per platform
+        UniqueConstraint("store_id", "platform_id", name="uq_store_domain_platform"),
         Index("idx_domain_active", "domain", "is_active"),
         Index("idx_store_domain_primary", "store_id", "is_primary"),
         Index("idx_store_domain_platform", "platform_id"),
diff --git a/app/modules/tenancy/models/store_platform.py b/app/modules/tenancy/models/store_platform.py
index 29a33506..6d53e99c 100644
--- a/app/modules/tenancy/models/store_platform.py
+++ b/app/modules/tenancy/models/store_platform.py
@@ -152,6 +152,12 @@ class StorePlatform(Base, TimestampMixin):
             "platform_id",
             name="uq_store_platform",
         ),
+        # Same custom_subdomain cannot be claimed twice on the same platform
+        UniqueConstraint(
+            "custom_subdomain",
+            "platform_id",
+            name="uq_custom_subdomain_platform",
+        ),
         # Performance indexes
         Index(
             "idx_store_platform_active",