Frankenstein's monster, commonly referred to as Frankenstein, is a fictional character that first appeared in Mary Shelley's 1818 novel Frankenstein; or, The Modern Prometheus as its main antagonist. Shelley's title compares the monster's creator, Victor Frankenstein, to the mythological character Prometheus, who fashioned humans out of clay and gave them fire.

In Shelley's Gothic story, Victor Frankenstein builds the creature in his laboratory through an ambiguous method based on a scientific principle he discovered. Shelley describes the monster as 8 feet (240 cm) tall and emotional. The monster attempts to fit into human society but is shunned, which leads him to seek revenge against Frankenstein. According to the scholar Joseph Carroll, the monster occupies "a border territory between the characteristics that typically define protagonists and antagonists".

Frankenstein's creature became iconic in popular culture, and has been featured in various forms of media, including films, television series, merchandise and video games. The most popularly recognized version is Boris Karloff's portrayal of the monster in the 1930s films Frankenstein, Bride of Frankenstein, and Son of Frankenstein.

Mary Shelley's original novel does not give the character a specific name. In the novel, Victor Frankenstein variously refers to his creation as the "creature", "fiend", "spectre", "dæmon", "wretch", "devil", "thing", "being", and "ogre". Frankenstein's creation referred to himself as a "monster" at least once, as did the residents of a hamlet who saw the creature towards the end of the novel.

As in Shelley's story, the creature's namelessness became a central part of the stage adaptations in London and Paris during the decades after the novel's first appearance. In 1823, Shelley herself attended a performance of Richard Brinsley Peake's Presumption, the first successful stage adaptation of her novel. "The play bill amused me extremely, for in the list of dramatis personae came, -------- by Mr T. Cooke," she wrote to her friend Leigh Hunt. "This nameless mode of naming the unnameable is rather good."

Within a decade of publication, the name of the creator, "Frankenstein", was used to refer to the creature, but it did not become firmly established until much later. The story was adapted for the stage in 1927 by Peggy Webling, and Webling's Victor Frankenstein does give the creature his name. However, the creature has no name in the Universal film series starring Boris Karloff during the 1930s, which was largely based upon Webling's play. The 1931 Universal film treated the creature's identity in a similar way as Shelley's novel: in the opening credits, the character is referred to merely as "The Monster" (the actor's name is replaced by a question mark, but Karloff is listed in the closing credits). However, in the sequel Bride of Frankenstein (1935), the frame narration by a character representing Shelley's friend Lord Byron does refer to the monster as Frankenstein. Nevertheless, the creature soon enough became best known in the popular imagination as "Frankenstein". This usage is sometimes considered erroneous, but some usage commentators regard the monster sense of "Frankenstein" as well-established and not an error.

Modern practice varies somewhat. For example, in Dean Koontz's Frankenstein, first published in 2004, the creature is named "Deucalion", after the character from Greek mythology, who is the son of the Titan Prometheus, a reference to the original novel's title. Another example is the 2014 film I, Frankenstein, where the Queen of the Gargoyles Leonore gives him the name "Adam" after understanding Victor never gave him one. The creature is also referred to as Adam in the 2017 novel The Strange Case of the Alchemist's Daughter by Theodora Goss. In the second episode of Showtime's Penny Dreadful, also from 2014, Victor Frankenstein briefly considers naming his creation "Adam", before deciding instead to let the monster "pick his own name". Thumbing through a book of the works of William Shakespeare, the monster chooses "Proteus" from The Two Gentlemen of Verona. It is later revealed that Proteus is actually the second monster Frankenstein has created, with the first, abandoned creation having been named "Caliban", from The Tempest, by the theatre actor who took him in and later, after leaving the theatre, named himself after the English poet John Clare. Another example is an attempt by Randall Munroe of webcomic xkcd to make "Frankenstein" the canonical name of the monster, by publishing a short derivative version which directly states that it is. The 2004 film Van Helsing, a Universal production, depicts updated versions of some of its characters, including the Monster who is addressed directly by the title character as "Frankenstein".

Victor Frankenstein builds the creature over a two-year period in the attic of his boarding house in Ingolstadt after discovering a scientific principle which allows him to create life from non-living matter. Frankenstein is disgusted by his creation, however, and flees from it in horror. Frightened, and unaware of his own identity, the monster wanders through the wilderness.