diff --git a/priv/repo/migrations/20260122231235_assign_mitglied_role_to_existing_users.exs b/priv/repo/migrations/20260122231235_assign_mitglied_role_to_existing_users.exs
new file mode 100644
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+++ b/priv/repo/migrations/20260122231235_assign_mitglied_role_to_existing_users.exs
@@ -0,0 +1,41 @@
+defmodule Mv.Repo.Migrations.AssignMitgliedRoleToExistingUsers do
+  @moduledoc """
+  Assigns the "Mitglied" role to all existing users without a role.
+
+  This migration runs once during deployment to ensure all users have a role assigned.
+  New users will automatically get the "Mitglied" role via the AssignDefaultRole change.
+  """
+  use Ecto.Migration
+  import Ecto.Query
+
+  def up do
+    # Find the Mitglied role ID
+    mitglied_role_id =
+      repo().one(
+        from(r in "roles",
+          where: r.name == "Mitglied",
+          select: r.id
+        )
+      )
+
+    if mitglied_role_id do
+      # Assign Mitglied role to all users without a role
+      updated_count =
+        repo().update_all(
+          from(u in "users", where: is_nil(u.role_id)),
+          set: [role_id: mitglied_role_id]
+        )
+
+      IO.puts("✅ Assigned 'Mitglied' role to #{updated_count} existing user(s)")
+    else
+      IO.puts("⚠️  Warning: 'Mitglied' role not found - skipping role assignment")
+      IO.puts("   This is expected if roles haven't been seeded yet.")
+      IO.puts("   New users will get the role automatically via AssignDefaultRole change.")
+    end
+  end
+
+  def down do
+    # Not reversible - we can't know which users had no role before
+    :ok
+  end
+end