A monster is a type of fictional creature found in horror, fantasy, science fiction, folklore, mythology and religion. Monsters are very often depicted as dangerous and aggressive with a strange, grotesque appearance that causes terror and fear. Monsters usually resemble bizarre, deformed, otherworldly and/or mutated animals or entirely unique creatures of varying sizes, but may also take a human form, such as mutants, ghosts and spirits, zombies or cannibals, among other things. They may or may not have supernatural powers, but are usually capable of killing or causing some form of destruction, threatening the social or moral order of the human world in the process.

Animal monsters are outside the moral order, but sometimes have their origin in some human violation of the moral law (e.g. in the Greek myth, Minos does not sacrifice to Poseidon the white bull which the god sent him, so as punishment Poseidon makes Minos' wife, Pasiphaë, fall in love with the bull. She copulates with the beast, and gives birth to the man with a bull's head, the Minotaur). Human monsters are those who by birth were never fully human (Medusa and her Gorgon sisters) or who through some supernatural or unnatural act lost their humanity (werewolves, Frankenstein's monster), and so who can no longer, or who never could, follow the moral law of human society.

Monsters may also be depicted as misunderstood and friendly creatures who frighten individuals away without wanting to, or may be so large, strong and clumsy that they cause unintentional damage or death. Some monsters in fiction are depicted as mischievous and boisterous but not necessarily threatening (such as a sly goblin), while others may be docile but prone to becoming angry or hungry, thus needing to be tamed and taught to resist savage urges, or killed if they cannot be handled or controlled successfully.

Monsters pre-date written history, and the academic study of the particular cultural notions expressed in a society's ideas of monsters is known as monstrophy. Monsters have appeared in literature and in feature-length films. Well-known monsters in fiction include Count Dracula, Frankenstein's monster, werewolves, vampires, demons, mummies, and zombies.