Juggernaut (Cain Marko) is a fictional character appearing in American comic books published by Marvel Comics. Created by writer Stan Lee and artist/co-writer Jack Kirby, he first appeared in X-Men #12 (July 1965) as an adversary of the eponymous superhero team. Since then, he has come into conflict with other heroes, primarily Spider-Man and the Hulk.

Cain Marko is a regular human who was empowered by a gem belonging to the deity Cyttorak, becoming a literal human juggernaut. He possesses superhuman strength and durability, and is virtually immune to most physical attacks; his helmet also protects him from mental attacks. Although not a mutant, Juggernaut has been featured as a prominent member of the Brotherhood of Mutants. He is also the stepbrother of Professor X.

Since his debut during the Silver Age of Comic Books, the character has appeared in over five decades of Marvel publications, featuring prominently in the X-Men titles and starring in two one-shot solo publications. The character has also been associated with Marvel merchandise including clothing, toys, trading cards, animated television series, video games. Vinnie Jones played Juggernaut in the 2006 film X-Men: The Last Stand, while Ryan Reynolds provided motion capture and voice acting for the CGI character in the 2018 film Deadpool 2. In some reinterpretations (most notably the X-Men film series), Marko is a mutant who was born with his powers, while in others they simply come from his costume.

In 2008, Juggernaut was ranked 188th on Wizard's list of Top 200 Comic Book Characters. In 2009, Juggernaut was ranked 19th on IGN's list of Top 100 Comic Book Villains. IGN also ranked him as Spider-Man's 22nd greatest enemy.