Morlocks are a group of mutant characters appearing in American comic books published by Marvel Comics. The characters are usually depicted as being associated with the X-Men in the Marvel Universe. Created by writer Chris Claremont and artist Paul Smith, they were named after the subterranean race of the same name in H. G. Wells' novel The Time Machine, but unlike in the Wells book, they are not a faceless, threatening mass of villains. They first appeared as a group in The Uncanny X-Men #169 (May 1983). Caliban appeared prior to that, but he was not yet a member of the Morlocks.

The Morlocks were depicted as an underground society (both literally and figuratively) of outcast mutants living as tunnel dwellers in the sewers, abandoned tunnels, and abandoned subway lines beneath New York City. The Morlocks were composed of mutant misfits, especially those mutants who, because of physical mutations or other conspicuous manifestations of their mutant genetics, were unable to pass as human in normal society. Subjected to hate, fear, and disgust from human society due to their "deformed" appearances, dangerous mutations, or otherwise outcast or misfit statuses, most of the Morlocks viewed humans (and even other more mainstream mutants such as the X-Men) with distrust and anger, and they occasionally committed criminal or antisocial acts upon the above-ground human society.

Due to a series of tragedies, the original Morlocks no longer resided in subterranean New York City (except Marrow, who was one of the original Morlocks as a child). However, a violent splinter cell, Gene Nation, and a comparable group, called Those Who Live in Darkness, emerged in their absence. Similar groups, referred to as Morlocks by both readers and the X-Men, have appeared beneath Chicago and London. In the post-Krakoan Age, the Morlocks have returned to New York City, reclaiming their place in the underground tunnels.

According to Callisto, she formed the Morlocks by first recruiting Caliban. She then used his power to track down other mutants who were unable to integrate into normal society. The Morlocks initially squatted in a network of abandoned, interconnected tunnels beneath Manhattan, which had originally been built as Cold War bomb shelters and then forgotten, before expanding into sewers and abandoned subways. The Morlocks occasionally emerged to rob humans in Manhattan and would sometimes kidnap certain mutant children.

The X-Men were alerted to the existence of the Morlocks when their leader Callisto kidnapped Angel and intended to make him her mate. This brought Storm to challenge Callisto to a duel for leadership of the Morlocks. Victorious, Storm orders an end to their attacks on normal humans, but she does not assume leadership of the Morlocks full-time. Against her wishes, several Morlocks later kidnap the child superheroes Power Pack so that they could be raised by the Morlock Annalee, who had lost her own children. When Callisto discovers what was going on, she forces Annalee to let them go. They promise to return in the future to keep her company.

A majority of the Morlocks are killed by Mister Sinister's Marauders in the "Mutant Massacre". Most of the survivors join Gene Nation, having lived in a pocket dimension led by Mikhail Rasputin. Leech instead becomes a ward of Generation X. Other survivors include Erg and Beautiful Dreamer (whose whereabouts were unknown until "Decimation"), Thornn, (who would join X-Corporation), and Caliban.

It is later revealed that many of the Morlocks were actually failed experiments of the Dark Beast, although he made certain that they didn’t remember being tampered with, which is why Mister Sinister sought to destroy them.

A new group called the Tunnel Rats which also calls itself "Those Who Live in Darkness" have inhabited the sewer tunnels just below the surface of Mutant Town/District X. District X writer David Hine claims to never have intended this group to have any ties to the original Morlocks.