James Buchanan "Bucky" Barnes is a character appearing in American comic books published by Marvel Comics. Originally introduced as a sidekick to Captain America, the character was created by Joe Simon and Jack Kirby and first appeared in Captain America Comics #1 (cover-dated March 1941) (which was published by Marvel's predecessor, Timely Comics). Barnes' original costume (or one based on it) and the Bucky nickname have been used by other heroes in the Marvel Universe over the years.

The character is brought back from supposed death as the brainwashed assassin cyborg called the Winter Soldier (created by Ed Brubaker and Steve Epting) (Russian: Зимний Солдат, translit. Zimniy Soldát). The character's memories and personality are later restored, leading him to become a dark hero in search of redemption. He temporarily assumes the role of Captain America when Steve Rogers was presumed to be dead. During the 2011 crossover Fear Itself, Barnes is injected with the Infinity Formula, which increases his natural vitality and physical traits in a way similar to (but less powerful than) the super-soldier serum used on Captain America. After coming into contact with the shadowy Outer Circle, Barnes becomes the new Revolution, bringing down the organization from within.

Bucky Barnes has been adapted into various media outside comics, including animated series and video games. Sebastian Stan portrays the character across multiple entries in the Marvel Cinematic Universe, including Captain America: The First Avenger (2011), Captain America: The Winter Soldier (2014), Ant-Man (2015), Captain America: Civil War (2016), Black Panther (2018), Avengers: Infinity War (2018), Avengers: Endgame (2019), the miniseries The Falcon and the Winter Soldier (2021), the animated series What If...? (2021-2024), Captain America: Brave New World (2025), Thunderbolts* (2025) and the upcoming Avengers: Doomsday (2026).

When Joe Simon created his initial sketch of Captain America for Marvel Comics precursor Timely Comics in 1940, he included a young sidekick. "The boy companion was simply named Bucky, after my friend Bucky Pierson, a star on our high school basketball team", Simon said in his autobiography. Following the character's debut in Captain America Comics #1 (March 1941), Bucky Barnes appeared alongside the title star in virtually every story in that publication and other Timely series, and was additionally part of the all-kid team the Young Allies. In the post-war era, with the popularity of superheroes fading, Bucky appeared alongside team-leader Captain America in the two published adventures of Timely/Marvel's first superhero group, the All-Winners Squad, in All Winners Comics #19 and #21 (Fall–Winter 1946; there was no issue #20).

After Bucky was shot and seriously wounded in Captain America Comics #66 (April 1948), he was succeeded by Captain America's girlfriend Betsy Ross, who became the superhero Golden Girl. Bucky recovered and was briefly reunited with Captain America for an appearance in Captain America Comics #71 (March 1949), but otherwise did not appear for the rest of the run. Captain America Comics ended with #75 (Feb. 1950), by which time the series had been titled Captain America's Weird Tales for two issues, with the finale a horror/suspense anthology issue with no superheroes.

Captain America and Bucky were both briefly revived, along with fellow Timely stars the Human Torch and the Sub-Mariner, in the omnibus Young Men #24 (Dec. 1953), published by Marvel's 1950s iteration Atlas Comics. Bucky appeared alongside "Captain America, Commie Smasher!", as the hero was cover-billed, in stories published during the next year in Young Men and Men's Adventures, as well as in three issues of Captain America that continued the old numbering. Sales were poor, however, and the series was discontinued with Captain America #78 (Sept. 1954).

Retroactive continuity, beginning with The Avengers #4 (March 1964), established that the original Captain America and Bucky went missing near the end of World War II and were secretly replaced by then-U.S. President Harry S. Truman with successor heroes using those identities. This retroactively meant that the Bucky who operated with the All-Winners Squad and was later wounded before being replaced by Golden Girl was a different hero (which also explained how Bucky could still be in his mid-teens years after his earliest adventures). Later comics said this second Bucky was a teenager named Fred Davis, and it was established that he and Bucky Barnes had met and befriended each other before the end of World War II. The 1950s version of Bucky was retroactively said to be Jack Monroe, a college student who had been inspired by the exploits of the original Captain America and Bucky Barnes. In the 1980s, Jack Monroe adopted the identity Nomad (an alias Steve Rogers briefly used during the 1970s when he was disillusioned with the U.S. government and the role of Captain America).

The original Bucky regularly co-starred with Captain America in flashback World War II adventures presented in Tales of Suspense #63–71 (March–Nov. 1965). Some of these stories were adaptations of Bucky and Cap's original 1940s stories published by Timely Comics, revising details to establish new canon for the Marvel Universe. Afterward, Bucky Barnes only occasionally appeared in further flashback stories or in dreams and memories that haunted Steve Rogers, who felt guilty for not preventing his death. For a brief time, 1960s stories depicted Rick Jones as Captain America's new sidekick in the modern-day, briefly given the costume and name of Bucky. During the 1980s, Steve Rogers temporarily stepped down as Captain America and was replaced by John Walker, whose friend Lamar Hoskins then adopted the identity and name of Bucky. Lamar changed his name to Battlestar when it was pointed out that "buck" has been a slur used against black men in parts of America.