As depicted in the film, Ariel is the youngest of Triton's seven daughters, and is 16 years old. She has a vast fascination with the world of humans, despite such contact being forbidden by Triton, who is xenophobic, meaning that he hates humans.
At the beginning of the film, Ariel and her fish friend, Flounder, swim through a shipwreck where they salvage human items. During their adventure, they are attacked by a shark, but Ariel manages to outwit him. After escaping him, Ariel and Flounder swim up to the surface, where they speak to Scuttle to identify human objects. He misidentifies a fork as a "dinglehopper" used to straighten their hair and a pipe as a "snarfblatt" used to make music. The identification of the latter reminds her about the concert she was initially supposed to a part of. Afterwards, she is watched by two eels, Flotsam and Jetsam. Their master, Ursula, watches from her crystal ball bubble and wants them to keep an extra close watch on her.
Ariel hurries home to Triton and Sebastian, who both rebuke her for missing the concert. Hearing from Flounder about her encounter with a seagull, Triton reprimands her for swimming up to the surface, telling her how she could have been spotted by what he calls "barbarians" who could have snared her with a fishhook. After being demanded by him to obey his every rule and not do that again, she tearfully swims off to her secret grotto, where her collection of human objects, from books to other regular objects such as globes, mugs, and chests of jewels, is kept. She feels that collecting them isn't enough and gets more curious about the human world, even expressing her desire to actually live in it.
Later, Ariel spots a ship passing above. Excited, she quickly swims up to the surface and toward it, where she watches the party on board. She then spots a male human named Eric, and remarks to Scuttle how she has never seen a human this close before. Just then, a storm moves in and destroys the ship. She looks around for Eric, and finds him floating unconscious in the sea. She carries him to the shore, where she sings to him about how she longs to be with him in his world, but is forced to leave when others approach. She falls in love with him, and becomes even more determined to accomplish her dreams of living alongside humanity.
Back at Atlantica, Ariel's other sisters notice her humming dreamily, giving them a notion that she is in love. She comes up with a plan to somehow see Eric again, but Sebastian tries to talk her out of it, telling her how life under the sea is better than the world up above through the song "Under the Sea". While he tries to entertain her with a band of crustaceans producing music, Flounder comes over to her, whispers something in her ear, and they both swim off.
Flounder takes Ariel to her grotto, where he surprises her with a statue of Eric that he has somehow salvaged from the destroyed ship and added into her collection. She thanks him, but then faces the stern Triton, who is very upset to learn from Sebastian that she has rescued a human being from drowning. Shocked and enraged that she is in love with that human, he loses all his temper and proceeds to destroy all the human objects in her grotto with his trident, including the statue of Eric. She tries pleading for him to stop, but to no avail. Devastated, she breaks down in tears, and an ashamed Triton leaves regretting what he has done. Sebastian, who has accompanied him, try to console her, only for her to tell him to leave. After he and Flounder exit, two eels, Flotsam and Jetsam, appear to convince her to see their mistress, Ursula, who may have the power to make her dream come true. She, after initially attempting to refuse, reluctantly follows them to Ursula's fortress after the eels subtly flicked what remained of Eric's statue toward her while "taking their leave".
Ariel nervously and fearfully enters the fortress, and Ursula calls for her. Ursula, who knows that she is in love with a human through spying on her via Flotsam and Jetsam, tells her that she must become a human herself to be with Eric. Ursula explains the deal to her: she will become a human for three days, during which she must get a kiss of true love from him before the sun sets on the third day in order to stay human; otherwise, she will turn back into a mermaid and will belong to Ursula forever. Ursula further states that as part of the deal, she must give up her voice in order to become a human. (Unbeknownst to her, the deal is simply a part of Ursula's sinister plot to take Triton's throne.) She, thinking that is her only chance to be with Prince Eric, accepts the deal as she signs a contract conjured by Ursula, then orders Ariel to sing for her and summons magical hands to rip out her voice, which is then sealed into a seashell on Ursula's necklace. Ariel is then turned into a human, and is helped to the surface by Flounder and Sebastian due to her inability to swim and breathe.
At the shore, Scuttle arrives and witnesses Ariel's new legs for the first time. However, Sebastian is beyond furious, going into a nervous panic at what would happen if she failed to kiss Eric, as well as if King Triton were to find out about the entire event. Frustrated, he begins trotting home, only to have her convince him to stay and help, knowing if they reversed the curse and went back home, she would be miserable once again. Now that the mission is set in motion, Scuttle provides some "clothing" for her, just as they are found by Eric's dog, Max, who leads his master to her. When he sees her, he finds her familiar, but when he sees that she is unable to talk, he gets the feeling that she can't be the girl he is looking for. Nevertheless, he takes her to his castle, where she takes a bath and gets dressed for dinner. As he, his manservant, Grimsby, and maid Carlotta, join her, she notices a fork and uses it to comb her hair, making them dumbfounded, which makes her embarrassed. Noticing Grimsby's pipe, she takes it and, thinking it as a musical instrument, blows the contents into his face, and Eric laughs at him covered with ashes. After he wipes it off and dinner is served, he suggests that Eric take her out on a tour of his kingdom, which she accepts.
Vanessa transforms back into Ursula, takes Ariel as her prisoner, and dives back into the sea with her. As Ursula thinks of her next move, Triton having been alerted by Sebastian, confronts and orders her to release Ariel, to which she refuses. Ariel apologizes to Triton as he attempts to destroy the contract, to no avail. Ursula proceeds to wither the princess, proclaiming the contract to be legally unbreakable, but offers her freedom should Triton be willing to take her place. Triton accepts, and though Ariel is released, she is forced to watch as her father is withered and transformed into a polyp. Ursula then takes his crown and trident, becoming queen of the seas, moments before an enraged Ariel tries to fight her off, though she is quickly restrained. Eric arrives to help her, striking Ursula with a harpoon. She prepares to destroy him with the trident, but Ariel pulls on her hair just as it shoots, causing the shot to miss Eric and hit Flotsam and Jetsam instead, killing them.
After Ursula shortly mourns her pets' loss, she enlarges herself as an even more evil, nastier, monstrous version. Ariel and Eric try to escape, but then face a towering, gigantic Ursula. She declares herself as ruler of the ocean who has the power to control the waves and creates a rainstorm and whirlpool. Ariel is separated from Eric, and Ursula causes her to become trapped at the bottom of the whirlpool. She aims the trident at Ariel and fires bolts of pure destruction, who barely manages to avoid her painful destruction each time. Just as Ursula prepares to use the trident to destroy her with an unavoidable blast and taunting that she will never kiss Eric, he plunges the splintered bow of one of the shipwrecks (that have surfaced as a result of the whirlpool) through her abdomen, killing her and saving Ariel at the last minute.
With Ursula defeated, her spells upon the unfortunate merpeople are broken, including the spell cast upon Triton, who transforms back into a merman. At the surface, Eric manages to swim to shore, breaking down out of exhaustion as a saddened Ariel watches on from a nearby rock. Triton and Sebastian look on from a distance; the former finally softening up toward humans upon realizing Eric risked his own life to save Ariel. Knowing the love between them is pure and true, despite their differences as species, Triton uses the power of the trident to permanently transform Ariel into a human, allowing her to be with the one she loves, at long last. After the transformation, she walks up to the shore, where she is happily reunited with Eric as he embraces her, and they share their first kiss. Soon, they have their wedding on the wedding ship attended by his loyal subjects onboard and her family and friends in the sea around the ship. She silently bids farewell to Scuttle and Flounder and gives Triton a heartwarming hug before happily sailing away with Eric. Thanks to her, Triton accepts Eric, her new husband, as his new son-in-law.
- Since her father, King Triton, is the son of Poseidon and the cousin of Hercules, that would mean that she is Hercules' niece and Hades' great niece.
- She was voiced by Jodi Benson in her debut.
- She is the first Disney Princess to have red hair. Merida is the second.
- In School of Secrets, it's revealed that she is the aunt of a girl (who is the daughter of one of her sisters).
- If she was 16 years old in her debut, The Little Mermaid, and the beginning of Auradon was 20 years ago, that means that she is now 36 years old.
- She brought back the Disney Princesses. When Walt Disney died, only Snow White, Cinderella and Aurora existed; they didn't make a new fairy tales-based movie until 30 years later.
- By this, she began the renaissance of Disney.