Leonardo, the Terrible Monster (known on the front cover as Your Pal Mo Willems Presents: Leonardo, the Terrible Monster) is a children's picture book written and illustrated by Mo Willems.

[1] In 2007, Weston Woods adapted the book to an animated film, directed by Pete List.

Kirkus Reviews called it a "sweetly original morality play about a very unscary monster",[3] while Publishers Weekly wrote " Willems's (Don't Let the Pigeon Drive the Bus! )

finale feels apt but syrupy; Leonardo's decision to be nice seems homiletic.

Yet this is an appealing book, sketched in dark brown against grayish pastel backdrops, with evergreen lettering and highlighted keywords.