Marinette's magic box is a wooden box that Marinette Dupain-Cheng made to store her diary. It's purpose is to prevent anyone from stealing the diary and discovering important secrets, such as her being Ladybug.
The magic box is a semi-circular chest with a similar shape and color theme as the diary with a keyhole located on the edge. Inside is colored red with an oval-shaped mirror on the top half in the center. The chest comes with a yellow key with a red ribbon that Marinette carries with her. Its lock is designed similar to a ladybug but with a pink shell instead of red.
If someone tries to pick up Marinette's diary while the chest is open, it will close and lock, trapping the invader's hand inside. The only way to unlock it is with a key that Marinette keeps with her.