Bartholomew Henry "Bart" Allen II is a superhero appearing in American comic books published by DC Comics. A speedster, he first appeared under the alias Impulse and later became the second Kid Flash and the fourth Flash. Created by Mark Waid and Mike Wieringo, Bart first made a cameo in The Flash (vol. 2) #91 in 1994 before his full debut in issue #92. He has since been featured as the lead character in Impulse (1995–2002) and The Flash: The Fastest Man Alive (2006–2007). Bart also appears in the series Young Justice and Teen Titans as a member of both superhero teams.

As first conceived by writers, Bart was born in the 30th century to Meloni Thawne and Don Allen, and is part of a complex family tree of superheroes and supervillains. His father, Don, is one of the Tornado Twins and his paternal grandfather is Barry Allen, the second Flash. His paternal grandmother, Iris West, is also the adoptive aunt of the third Flash, Wally West (Bart's first cousin once removed). Additionally, Bart is the first cousin of XS, a Legionnaire and daughter of Dawn Allen. On his mother's side, he is a descendant of supervillains Professor Zoom and Cobalt Blue as well as the half-brother of Owen Mercer, the second Captain Boomerang. In addition to these relatives, he had a supervillain clone known as Inertia.

For most of his superhero career, Bart was the teenage sidekick to Wally West. After West's apparent death in the Infinite Crisis crossover event in 2006, Bart became the Flash. His tenure as the Flash was brief and concluded with his death in issue 13 of The Flash: The Fastest Man Alive. Bart remained dead for two years of publication before being resurrected in 2009's Final Crisis: Legion of 3 Worlds. During DC's The New 52 era, Bart Allen was reintroduced in Teen Titans as the alias of an amnesiac revolutionary from the future named Bar Torr; the character was later written out of comics, and his tenure erased from continuity by the subsequent DC Rebirth initiative. The original Bart Allen was brought back as part of DC Rebirth's Wonder Comics Young Justice series.

Outside of comics, Bart has been portrayed by Kyle Gallner in the live-action television series Smallville. Jason Marsden voiced the character in the animated series Young Justice. Jordan Fisher portrays a variation of the character as the future son of Barry Allen and Iris West-Allen starting in the seventh season of The CW Arrowverse television series The Flash as a recurring character.

Bart Allen is the son of Meloni Thawne (a descendant of Eobard Thawne) and Don Allen (the son of Barry Allen). Don and his sister Dawn were killed in an invasion by the Dominators when Bart was an infant. Bart was born with full super-speed; his cousin Jenni Ognats, daughter of Dawn Allen and Jeven Ognats, did not at first display any signs of super-speed. At the sight of seeing her father tortured, her latent super-speed powers activated and would go on to join the Legion under the name XS. It is later revealed that these events took place on the parallel world of Earth-247, with Don, Dawn, and their families having traveled there from the main universe.

Due to his Speed Force abilities, Bart undergoes rapid aging, causing him to appear to be twelve when he is chronologically only two years old. To prevent him from developing mental health problems, he is raised in a virtual reality world that adheres to his sense of time. When it becomes clear that this method is not working, Bart's grandmother Iris West takes him to the 21st century, where Wally West tricks Bart into overexerting his abilities and returning his metabolism to normal. Because he spent most of his childhood in a simulated world, Bart has no concept of danger and is prone to reckless behavior. Bart proves to be more trouble than Wally can handle and is placed in the custody of Max Mercury, who moves with him to Manchester, Alabama.

Bart joins the Teen Titans early in his career before becoming a founding member of the superhero team Young Justice alongside Robin and Superboy. Impulse stays with Young Justice for an extensive period of time, during which he develops the ability to create Speed Force energy duplicates. One of Bart's clones is killed during the "Our Worlds at War" storyline, causing Bart to quit Young Justice temporarily as he comes to terms with his mortality.

Following Max Mercury's disappearance, Bart is adopted by Jay Garrick, the first Flash, and his wife Joan. After Young Justice disbands, Bart joins some of his former teammates in the Teen Titans.