Nightmare is a supervillain appearing in American comic books published by Marvel Comics. He first appeared in Strange Tales #110 and was created by Stan Lee and Steve Ditko. The character is depicted most commonly as a major enemy of Doctor Strange and Ghost Rider. Nightmare is the ruler of a Dream Dimension and he is one of the Fear Lords. He is also part of the group called The Six Fingered Hand. He can drain the psychic energies from the subconscious minds of dreaming beings.

Nightmare first appeared in Strange Tales #110 (the first appearance of his enemy, Dr. Strange) and was created by Stan Lee and Steve Ditko.

Nightmare is the evil ruler of the "Dream Dimension", where tormented humans are brought during their sleep. He roams this realm on his demonic black horned horse, Dreamstalker. In physical form, he appears as a chalk-white man with green hair, a green suit, and a ragged cape. He was the first foe met by Doctor Strange when a man who was having troubling dreams went to Strange for help, though it turned out to be repressed guilt for an act of murder. Nightmare then traps Strange in his realm and tries to feed on his power. However, Strange escapes the Dream Dimension after tricking Nightmare with an illusion of his enemies.

Nightmare is a necessary part of human existence; without him to regulate fear through dreaming, humans would slowly be driven insane. At one point, Strange is forced to aid his enemy to prevent the latter's death from an inability to create new dreams.

Nightmare also serves under Shuma-Gorath and warned Strange that the demon would be a force that even the Sorcerer Supreme would have trouble defeating, and he once joined the Fear Lords, a group of supernatural creatures who fed on fear, to attack Strange together. Their plans are undone when D'Spayre tricks him into competing with the Dweller-in-Darkness over who could frighten humanity more; recognizing that this would only allow D'Spayre to surpass them both in power, the two entities call a truce.

Nightmare is the father of Dreamqueen, a similar being who rules her own "dream dimension". She was conceived when Nightmare impregnated a succubus named Zhilla Char.

Nightmare's realm is not part of the Mindscape, but the Sleepwalkers are aware of him and consider him an enemy. Because Sleepwalkers do not have to sleep, Nightmare is unable to affect them. He seeks to do so by tormenting Rick Sheridan, the host of an individual Sleepwalker who had been trapped in his mind. Nightmare attempts to convince Sleepwalker that Rick is safe, but he is not fooled and sacrifices his chance to return to the Mindscape to stop Nightmare.

In the Tempest Fugit storyline of The Incredible Hulk, it is revealed that Nightmare has been plaguing the Hulk for years with hallucinations, misdirections, and manipulations of reality, by empowering himself. His second, more benevolent, daughter Daydream is also introduced in this story arc. Nightmare claimed that this daughter was conceived by forcibly entering the mind of the Hulk's late wife, Betty Ross and raping her in her sleep. The Hulk kills Nightmare in retaliation, but he later returns to life.