Monsters vs. Aliens is a 2009 American animated science fiction action comedy film produced by DreamWorks Animation and distributed by Paramount Pictures. The film was directed by Conrad Vernon and Rob Letterman from a screenplay written by Letterman, Maya Forbes, Wallace Wolodarsky, and the writing team of Jonathan Aibel and Glenn Berger based on a story by Vernon and Letterman. Featuring the voices of Reese Witherspoon, Seth Rogen, Hugh Laurie, Will Arnett, Kiefer Sutherland, Rainn Wilson, Paul Rudd, and Stephen Colbert, the film involves a group of misfit monsters hired by the United States Armed Forces to stop the invasion of an extraterrestrial villain and save the world in exchange for freedom.

It was DreamWorks Animation's first feature film to be directly produced in a stereoscopic 3D format instead of being converted into 3D after completion, which added $15 million to the film's budget.

Originally slated for release on May 15, 2009, Monsters vs. Aliens was released on March 27, 2009, in the United States in 2D, RealD 3D, IMAX 3D, and 4DX. The film received generally positive reviews from critics and grossed $381 million worldwide on a $175 million budget. The film was the start to the Monsters vs. Aliens franchise, and despite not being followed by a sequel, it was followed by two television specials, Mutant Pumpkins from Outer Space and Night of the Living Carrots, and a Nickelodeon television series in 2013, that all take place after the film.