diff --git a/backend/open_webui/migrations/versions/f0bd01a18a3d_add_unique_normalized_user_email_index.py b/backend/open_webui/migrations/versions/f0bd01a18a3d_add_unique_normalized_user_email_index.py
new file mode 100644
index 0000000000..4281317110
--- /dev/null
+++ b/backend/open_webui/migrations/versions/f0bd01a18a3d_add_unique_normalized_user_email_index.py
@@ -0,0 +1,83 @@
+"""add unique normalized user email index
+
+Revision ID: f0bd01a18a3d
+Revises: 959eaac8f909
+Create Date: 2026-07-27 04:41:12.708743
+
+"""
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+from alembic import context, op
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'f0bd01a18a3d'
+down_revision: str | None = '959eaac8f909'
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+INDEX_NAME = 'uq_user_email_lower'
+EMAIL_IS_NOT_NULL = sa.text('email IS NOT NULL')
+LOWER_EMAIL = sa.text('lower(email)')
+
+
+def _index_exists() -> bool:
+    conn = op.get_bind()
+    inspector = sa.inspect(conn)
+    return INDEX_NAME in {index['name'] for index in inspector.get_indexes('user')}
+
+
+def _duplicate_emails() -> list:
+    conn = op.get_bind()
+    return conn.execute(
+        sa.text(
+            """
+            SELECT lower(email) AS email, count(*) AS duplicate_count
+            FROM "user"
+            WHERE email IS NOT NULL
+            GROUP BY lower(email)
+            HAVING count(*) > 1
+            ORDER BY lower(email)
+            """
+        )
+    ).fetchall()
+
+
+def _create_index() -> None:
+    op.create_index(
+        INDEX_NAME,
+        'user',
+        [LOWER_EMAIL],
+        unique=True,
+        postgresql_where=EMAIL_IS_NOT_NULL,
+        sqlite_where=EMAIL_IS_NOT_NULL,
+    )
+
+
+def upgrade() -> None:
+    if context.is_offline_mode():
+        _create_index()
+        return
+
+    if _index_exists():
+        return
+
+    duplicates = _duplicate_emails()
+    if duplicates:
+        details = ', '.join(f'{row.email} (x{row.duplicate_count})' for row in duplicates)
+        raise RuntimeError(
+            'Cannot add unique normalized user email index because duplicate emails exist: '
+            f'{details}. Merge or remove the duplicate users and rerun migrations.'
+        )
+
+    _create_index()
+
+
+def downgrade() -> None:
+    if context.is_offline_mode():
+        op.drop_index(INDEX_NAME, table_name='user')
+        return
+
+    if _index_exists():
+        op.drop_index(INDEX_NAME, table_name='user')