As depicted in the 1989 film, Ariel is the youngest of King Triton's seven daughters (being 16 years old). She is shown as being adventurous and curious about the world of humans, a fascination which angers her father as merfolk are forbidden from making contact with the human world. Her best friend in the film is a fish named Flounder, and she develops a close relationship with a crab named Sebastian. Ariel salvages human items and keeps them in a secret grotto as part of her collection.

In the film Ariel saves the life of and falls in love with a human prince named Eric. She visits the sea witch, Ursula, to trade her voice to be human for three days in order to get a chance to win Eric's heart, not realizing that this agreement is part of Ursula's bigger plan to trap Ariel's father, King Triton. Ariel almost manages to win the agreement by getting the "kiss of true love", but is stopped by Ursula's underhanded tactics. When the three day limit has passed, Ariel returns to mermaid form and is claimed by Ursula. King Triton trades himself for Ariel, enabling Ursula to claim the trident. In the battle that follows, Eric destroys Ursula by ramming a derelict ship's splintered prow through her enlarged torso. When the film ends, Ariel is transformed into a human permanently by King Triton's trident, and marries Prince Eric.

Ariel's theme song, which is referred to by Disney crew as the "I Want" song, is "Part of Your World", which she sings in her secret grotto proclaiming her fascination for human things. The song was originally going to be cut from the final film, due to the claim that it slowing the story down, but Howard Ashman reportedly fought to keep it in. A reprise of the song is featured a little later in the film when Ariel declares that she wants to become part of Prince Eric's world.