Flounder is a bright yellow and blue colored tropical fish (despite the name, he is not a flounder) and Ariel's best friend, voiced by Jason Marin in the 1989 film, who also provided vocals in-character for the tie-in music album Sebastian From The Little Mermaid. Flounder scares easily, and is prone to panicking under stressful situations like a single shark breaching the sunken ship, but when Ariel is in trouble, he comes through for her without hesitation. In the film he is the only character to give unconditional support for Ariel's fascination with human things, and at one point gives Ariel a statue of Eric as a gift.

Flounder appears in all the episodes of the prequel television series, sharing constant adventures with Ariel and in the episode The Evil Manta shown how he first met Ariel when they were children. According to the television series, Flounder's real name is Guppy Number 35. In the series, he is voiced by Edan Gross and Bradley Pierce.

Flounder also appears in Jim Henson's Little Mermaid's Island where he has a twin sister named Sandy and is voiced by Veronica Taylor.

Flounder has a small role in The Little Mermaid II: Return to the Sea, where he is reunited with a grown-up Ariel and takes part in the search for Melody. He became a father himself, with five children of his own and he first meets Melody in Morgana's lair. In this film, he is voiced by Cam Clarke.

He has a larger role in The Little Mermaid: Ariel's Beginning, which shows an alternate version of how he first meets Ariel and later unknowingly leads her to the Catfish Club. His characterization is notably different in this movie; he does not scare as easily and is much more carefree and high-spirited. He performs beatboxing that kicks off a reprise of "Jump In The Line (Shake, Senora)" when he, Ariel, Sebastian and the Catfish Club Band are on the run from Atlantica. In this film, he is voiced by Parker Goris.

Flounder appears in the Kingdom Hearts series, where his role as Ariel's friend remains. His biggest role in the series is in Kingdom Hearts: Chain of Memories, where Ursula kidnapped him to force Ariel to give her the trident. He also sings his part in the song called "A New Day is Dawning". He also appears in Disney Princess: Enchanted Journey as Ariel's sidekick and is voiced by Anthony Skillman. He is also a remote-controlled playable character in the same game seen holding a big conch shell on his head with your help to catch the voice orbs.

At the Disney theme parks, Flounder makes cameo appearances in the Mickey's Philharmagic 3D show and in the Hong Kong Disneyland version of "It's a Small World". He has also appeared in the parks as a walk-around character, but mostly in parades, shows and special events such as "Mickey's Pirate & Princess Party".

Flounder appears in the stage adaptation of The Little Mermaid. His supporting role is similar as in the film, but he does not give Ariel the statue of Eric, and does not help Ariel reach Eric's wedding barge, as the Vanessa subplot has been removed. However, Flounder performs a new song titled "She's in Love", which he sings with Ariel's sisters when they notice that Ariel has been acting "fishy lately". The stage role was originated by Cody Hanford and J.J. Singleton, but the two actors had to leave the show when their height overshot that of Sierra Boggess, who originated Ariel. The role was taken over by Trevor Braun and Brian D'Addario. On the Original Broadway Cast Recording, Brian D'Addario performs as Flounder.