diff --git a/backend/open_webui/migrations/versions/1ce6ade7d93b_add_group_member_user_id_index.py b/backend/open_webui/migrations/versions/1ce6ade7d93b_add_group_member_user_id_index.py
new file mode 100644
index 0000000000..a5a00b7b2a
--- /dev/null
+++ b/backend/open_webui/migrations/versions/1ce6ade7d93b_add_group_member_user_id_index.py
@@ -0,0 +1,28 @@
+"""Add group_member user_id index
+
+Revision ID: 1ce6ade7d93b
+Revises: f0bd01a18a3d
+Create Date: 2026-07-31 03:00:00.000000
+
+"""
+
+import sqlalchemy as sa
+from alembic import op
+
+revision = '1ce6ade7d93b'
+down_revision = 'f0bd01a18a3d'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    conn = op.get_bind()
+    inspector = sa.inspect(conn)
+    existing_indexes = {idx['name'] for idx in inspector.get_indexes('group_member')}
+
+    if 'ix_group_member_user_id_group_id' not in existing_indexes:
+        op.create_index('ix_group_member_user_id_group_id', 'group_member', ['user_id', 'group_id'])
+
+
+def downgrade():
+    op.drop_index('ix_group_member_user_id_group_id', table_name='group_member')
diff --git a/backend/open_webui/models/groups.py b/backend/open_webui/models/groups.py
index 8f402f0b12..611afe576d 100644
--- a/backend/open_webui/models/groups.py
+++ b/backend/open_webui/models/groups.py
@@ -12,6 +12,7 @@ from sqlalchemy import (
     BigInteger,
     Column,
     ForeignKey,
+    Index,
     String,
     Text,
     and_,
@@ -71,6 +72,8 @@ class GroupModel(BaseModel):
 
 class GroupMember(Base):
     __tablename__ = 'group_member'
+    # The table's (group_id, user_id) unique constraint cannot serve user_id lookups.
+    __table_args__ = (Index('ix_group_member_user_id_group_id', 'user_id', 'group_id'),)
 
     id = Column(Text, unique=True, primary_key=True)
     group_id = Column(