April O'Neil is a fictional character from the Teenage Mutant Ninja Turtles comics. She is the first human ally of the Ninja Turtles.

April made her first appearance in the Mirage comic series in 1984 as a computer programmer. She was later portrayed as a strong-willed news reporter in the Turtles' first animated series, as a warrior in the Teenage Mutant Ninja Turtles Adventures comic produced by Archie Comics, and various other personas in different TMNT media.

While depicted as a red-haired adult woman in the comic series and the 1987 and 2003 animated series, she is depicted as being a teenager in the 2012 and 2018 animated series, 2022 film, and the 2023 film Mutant Mayhem. Her love interests have varied, though she is typically paired with the vigilante Casey Jones.

April was voiced by Renae Jacobs in the 1987 animated series, Veronica Taylor in the 2003 animated series, Sarah Michelle Gellar in the 2007 film TMNT, Mae Whitman in the 2012 animated series, Kat Graham in the 2018 animated series and its 2022 film, and Ayo Edebiri in the 2023 film Mutant Mayhem. In film, she has been portrayed by Judith Hoag (1990), Paige Turco (1991 and 1993), Megan Fox (2014 and 2016), and by Malina Weissman as the younger version of the character in the 2014 film.

In the original Mirage Comics storyline for Teenage Mutant Ninja Turtles, April O'Neil was a skilled computer programmer and assistant to a famous yet nefarious scientist, Baxter Stockman. She helped program his MOUSER robots but, after discovering Baxter was using them to burrow into bank vaults, she fled his workshop. Robots chased her into the sewer where she was promptly saved by three of the Turtles. The Turtles later successfully fended off a MOUSER invasion.

After leaving her job with Baxter, April decided to open an antique shop. The shop was subsequently destroyed in a battle between the Turtles and Shredder and the Foot Clan. April and the Turtles retreated to a farm house in Northampton, Massachusetts, to recover and during this time she suffered recurrent nightmares about the Foot Clan's attack. During the mid-1990s, April became romantically involved with the violent vigilante Casey Jones, and the two of them eventually raised Shadow, the child of Casey's late wife Gabrielle, as their own.

In Volume 2 of the TMNT comics, April was attacked by a huge robot controlled by the brain of her former boss, Baxter Stockman, and injected with nanobots. With the help of the Utroms, the Turtles injected April with turtle versions of nanobots to stop Baxter's plan. The intervention saved April before Baxter's nanobots could reach her brain stem and kill her. The attack rendered April sterile. To deal with the emotional strain she became a female version of "Nobody", a vigilante crime fighter, until her identity was discovered by Casey Jones. With the help of Renet, a time-traveler who took April back through time, it was revealed that April was really a living drawing brought to life with the help of a magic crystal. She was drawn by her father before his own biological daughter Robyn O'Neil was born. Although anything that the crystal's prior owner, Kirby, drew with pencil would vanish after some time, April's father used a pen, which might explain why April lived past thirty without vanishing. Questions of realness and morality were too much for April; she bid farewell to Shadow and Casey and travelled to Alaska to be alone with her thoughts. Although the trip helped April cope with her demons and led to her eventual return to New York, her family history remained unexplored.

In the Teenage Mutant Ninja Turtles Adventures series produced by Archie Comics, April began as a carbon copy of her animated counterpart from the 1987 TV series, but the writers developed her into a competent warrior after training with Splinter. Because of her frequent adventures with the Turtles, she lost her job at Channel 6 and became a freelance reporter.