diff --git a/backend/app/routers/todos.py b/backend/app/routers/todos.py
index 99c3cf5..1235d49 100644
--- a/backend/app/routers/todos.py
+++ b/backend/app/routers/todos.py
@@ -192,6 +192,22 @@ async def update_todo(
     for key, value in update_data.items():
         setattr(todo, key, value)
 
+    # Recalculate recurrence schedule if the todo is completed and now
+    # has a recurrence rule (e.g. user edited a completed todo to add
+    # recurrence, or changed the rule/due_date while completed).
+    if todo.completed and todo.recurrence_rule:
+        reset_at, next_due = _calculate_recurrence(
+            todo.recurrence_rule,
+            todo.due_date,
+            current_user.first_day_of_week,
+        )
+        todo.reset_at = reset_at
+        todo.next_due_date = next_due
+    elif todo.completed and not todo.recurrence_rule:
+        # Recurrence removed while completed — clear schedule
+        todo.reset_at = None
+        todo.next_due_date = None
+
     await db.commit()
     await db.refresh(todo)