The American animated television franchise Adventure Time features a cast of fictional characters created by Pendleton Ward. The first series in the franchise, Adventure Time revolves around the adventures of protagonists Finn the Human (voiced by Jeremy Shada), a teenage human boy, and his best friend Jake the Dog (voiced by John DiMaggio), a dog with magical powers to change shape and grow and shrink at will. Finn and Jake live in the post-apocalyptic Land of Ooo, where they travel on various adventures. Along the way, they interact with the other main characters of the show: Princess Bubblegum (voiced by Hynden Walch), the Ice King, Simon Petrikov (voiced by Tom Kenny), and Marceline the Vampire Queen (voiced by Olivia Olson). The second series Distant Lands, follow these characters along with the Peppermint Butler "Pep" (voiced by Steve Little), while Fionna and Cake follows Fionna Campbell (voiced by Madeleine Martin), Cake the Cat (voiced by Roz Ryan), Prismo the Wishmaster (voiced by Sean Rohani), Gary Prince (voiced by Andrew Rannells), Marshall Lee (voiced by Donald Glover), and the Scarab (voiced by Kayleigh McKee), along with a returning Kenny as Simon Petrikov.

This list only includes the main characters of the show, as well as major recurring characters (otherwise known as supporting characters) who are featured across several episodes of the series. Characters which are credited as guest stars or one-off characters are not included.

Finn the Human (voiced by Jeremy Shada in the TV series and Zack Shada in the original pilot short) is a human boy who loves nothing more than going on adventures and saving the day. He wears a hat that covers his extremely long flowing yellow hair. He is prone to exclamations and outbursts such as "Mathematical!" and "Algebraic!" When Finn was a baby, he was abandoned and subsequently adopted by Jake's parents, Joshua and Margaret, as detailed in "Memories of Boom Boom Mountain". Considering himself a hero, Finn has a lust for adventure and swore long ago that he would help anyone in need, but being so full of energy, he has trouble in situations that require him to do tasks other than fighting. After initially having an unrequited crush on Princess Bubblegum, Finn began a relationship with Flame Princess that lasted until the events of "Frost & Fire". In "Billy's Bucket List", it is revealed that Finn's birth father, Martin, is alive, and is trapped in a dimension known as the Citadel, which is a prison for the most dangerous criminals in the Multiverse. The actions of Finn and Jake, along with those of the Lich, cause Martin to be released; Finn soon learns that he is a petty criminal, and in the ensuing chaos, Finn loses his right arm, but later regains it in "Breezy" only to lose it again in "Reboot" before being fitted with a mechanical arm which can transform into many different weapons. The grass sword curse later re-manifests itself in the sixth-season finale, "The Comet", wherein Finn also learns that his spirit is the same as the catalyst comet.

Jake the Dog (voiced by John DiMaggio) is Finn's best friend and adoptive brother. He is a 28-year-old—in "magical dog years"—shapeshifting bulldog. The episode "Joshua & Margaret Investigations" reveals that when Joshua and Margaret—Finn and Jake's parents—were on an investigation, Joshua was bitten by a shape-shifting, other-worldly entity. Joshua, infected by the creature's venom, subsequently birthed Jake from his head. Jake's magic powers allow him to stretch or shrink any part of his body to any shape and size, ranging from becoming gigantic to becoming incredibly small. His powers help Finn considerably in combat and transportation, but are also sometimes used as nothing more than jovial forms of expression. Acting as a confidant and mentor to his energetic brother, Jake has a laid-back attitude in most situations, but loves adventure and will eagerly fight when he needs to do so. Jake is in a relationship with Lady Rainicorn, and they have five children together as seen in the episode "Jake the Dad". The two were drawn to each other due to their shared interest in playing the viola.

Jake passes away some time after the show's finale, "Come Along With Me", and, as shown in Adventure Time: Distant Lands and Adventure Time: Fionna and Cake, is immortalized by Finn via a tattoo on his chest.

Princess Bubblegum (voiced by Hynden Walch in the TV series and Paige Moss in the original pilot short) is a bubblegum humanoid, comparable to the inhabitants of the Candy Kingdom which she rules. The official series website has described her as "a millionaire nerd enthusiast [who] immerses herself in every branch of geekdom". She and Finn have a complex relationship. For the longest time Finn had a crush on Bubblegum, and although she cares for him a great deal, she did not return his feelings for him. In the episode "What was Missing", it is implied that she and Marceline may have had some sort of relationship in the past. This later caused an online controversy over her and Marceline's sexual orientation. In the second-season finale "Mortal Recoil", after being possessed by The Lich, she was accidentally shattered and returned to life as a 13-year-old due to the doctors' not having enough gum to rebuild her to her proper age, though it appears that her memories have remained intact. In the episode "Too Young", she becomes 18 again by absorbing the parts sacrificed by her candy subjects in order to reclaim her kingdom from the Earl of Lemongrab. After her brush with death, she has begun to feel more and more vulnerable. As a result, she created the clone-Sphinx named Goliad to serve as her heir lest she die on the throne. During the fifth and sixth seasons, it was gradually revealed that Bubblegum had an elaborate spy network, allowing her to monitor nearly everyone in Ooo. After growing increasingly darker and more devious, Bubblegum's Machiavellian actions were finally confronted by Flame Princess in "The Cooler", and since then, Bubblegum has made a concerted effort to relax her need to control everything. In the two-part sixth-season finale, Bubblegum is peacefully deposed after the candy citizens vote to replace her with the King of Ooo (voiced by Andy Daly). Rather than fight her rival, Bubblegum willingly goes into exile with Peppermint Butler until returning to the Candy Kingdom in the aftermath of the Stakes miniseries.

The Ice King (voiced by Tom Kenny in the TV series and John Kassir in the original pilot short) is a recurring antagonist-turned-supporting protagonist of the series, and is 1,043 years old. The Ice King frequently steals princesses throughout Ooo to forcefully marry them, Princess Bubblegum being his usual target. His ice-based magic abilities come from a magical crown he wears, which directly causes his insanity. The sixth season episode "Evergreen" reveals that the crown was created millions of years prior to the start of the series by an ice elemental named Evergreen in order to stop a comet from destroying all life on the planet. Though Ice King is defined as completely crazy by many, he is actually lonely and misunderstood. Furthermore, he is secretly envious of Finn and Jake for being such good friends.

Finn and Jake learned during the events of "Holly Jolly Secrets" that the Ice King was originally a human antiquarian named Simon Petrikov who bought his crown from a dock worker in northern Scandinavia, predating the Mushroom War. Wearing the crown, Petrikov began to lose both his mind and then his fiancée Betty; this explains his subconscious need for princesses. He soon began deteriorating in both mind and body over the years into his current state. Sometime before the Mushroom War he also discovered the Enchiridion. 996 years prior to the events of the series and directly after the Mushroom War, he met, befriended and cared for young Marceline. Eventually, he realized that his deteriorating mind and behavior would possibly become a threat to young Marceline. Thus, he wrote a letter to Marceline, describing why he could not help her anymore and imploring her to forgive him for whatever wrong he might do with the crown possessing him.