The third season of American animated television series Adventure Time, created by Pendleton Ward, originally aired on Cartoon Network in the United States. The series is based on a short produced for Frederator's Nicktoons Network animation incubator series Random! Cartoons. The season debuted on July 11, 2011, and the season finale was aired on February 13, 2012. The season follows the adventures of Finn, a human boy, and his best friend Jake, a dog with the magical powers to change shape, grow, and shrink at will. Finn and Jake live in the post-apocalyptic Land of Ooo. Along the way, they interact with the other main characters of the show: Princess Bubblegum, The Ice King, and Marceline the Vampire Queen.

The first episode of the season, "Conquest of Cuteness" was watched by 2.686 million viewers; this marked an increase in viewers watching Cartoon Network when compared to the previous season's debut. The season ended with the cliffhanger "Incendium", which was resolved at the start of season four. The season was met with largely positive critical reception. In addition, several episodes and writers were nominated for awards; the episode "Thank You" was nominated for an Annie Award as well as an award at the Sundance Film Festival. "Too Young" was nominated for an Emmy Award. Storyboard artist Rebecca Sugar was also nominated for an Annie Award. The tenth episode, however, garnered controversy from some parties because of an implied past lesbian relationship between Marceline and Princess Bubblegum.

Several compilation DVDs that contained episodes from the season were released after the season finished airing. The season was storyboarded and written by Ako Castuera, Tom Herpich, Adam Muto, Sugar, Jesse Moynihan, Bert Youn, Kent Osborne, Somvilay Xayaphone, Pendleton Ward, and Natasha Allegri while being produced by Cartoon Network Studios and Frederator Studios. The series is rated TV-PG.