Jack of Hearts (Jack Hart) is a fictional superhero appearing in American comic books published by Marvel Comics. The character first appeared in The Deadly Hands of Kung Fu #22 (March 1976), and was created by writer Bill Mantlo and artist Keith Giffen. He starred in his own four-issue mini series and has been a regular character in the Iron Man comics as well as at one point being a member of the Avengers.

Jack of Hearts is the son of scientist Philip Hart (creator of "Zero Fluid") and an alien woman from the Contraxian race. As a young man, Jack was doused in Zero Fluid and gained super powers which he could not control. In an attempt to control his powers, Jack of Hearts built a containment suit that resembled the Jack of Hearts playing card. The lack of control became a recurring theme with Jack of Hearts, including his stint in the Avengers where he had to spend 14 hours a day in a containment room to prevent self-destruction. When his powers became uncontrollable he chose to leave Earth, exploding in space without harming any innocent bystanders.

The return of Jack of Hearts and the explosion that killed Scott Lang was the launching point of the 2004-05 "Avengers Disassembled". The character was resurrected as part of one of the Marvel Zombies mini-series where Zero Energy reforms into Jack of Hearts, bringing the character back to life. The Zombies were later defeated with the help of Jack of Hearts.

Jack of Hearts first appeared in the black and white magazine The Deadly Hands of Kung Fu #22 (March 1976), and was created by writer Bill Mantlo and illustrator Keith Giffen. He was presented as an antagonist for the superhero White Tiger but his origin story in the following issue established him as a hero. His debut appearance in a color comic book was in The Incredible Hulk vol. 2 #214 (Aug. 1977) where he fights the Hulk. He later serves a brief stint as an ally and apprentice of Iron Man, becoming a regular cast member of the series during Mantlo's tenure as writer.

Jack has made various guest appearances in several books over the years, including Marvel Two-in-One, The Defenders, Marvel Premiere, (in a solo story meant to launch the character in his own series) ROM, and Marvel Team-Up, (which served as an intro to his 1984 mini-series) among others. He starred in his own self-titled, four issue limited series from January to April 1984.

He was later made a regular in The Avengers by writer Kurt Busiek, who explained, "I first ran into Jack in The Deadly Hands of Kung Fu in an issue drawn by Gil Kane and Rico Rival. That great overcomplicated costume and the energy that Gil put into the storytelling hooked me, and I’ve been a fan ever since".

In issue #40 of the 6th Fantastic Four series (dated February 2022), the character joined the titular team as a replacement for the Human Torch.

Jack Hart was born in New Haven, Connecticut. He is the son of Philip Hart, a scientist who created an energy source called "zero fluid", and Marie, a Contraxian alien. After being exposed to a vat of zero fluid, Jack underwent a mutagenic change and gained the ability to project concussive energy from his body. After gaining these powers, Jack began his mission of vengeance against the Corporation that killed his father. This led to Jack fighting other superheroes such as the Sons of the Tiger and the Hulk. He also fought the White Tiger. Eventually, he killed Stryke, his father's killer.