Mrs. Barnum is either a parent or a teacher at Westmore Middle School. She appears only during The Ugly Truth, in which she is a chaperone at the Lock-In. As Greg Heffley is bored, he asks Mrs. Barnum what to do. She tells him there is an "activity center" which consists of a ball pit and coloring books.
- Diary of a Wimpy Kid: The Ugly Truth (First and only appearance)