Frankenstein's monster or Frankenstein's creature, often erroneously referred to as simply "Frankenstein", is a fictional character who first appeared in Mary Shelley's 1818 novel Frankenstein; or, The Modern Prometheus as the main antagonist. Shelley's title thus 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 monster 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 versions are the film portrayals by Boris Karloff in the 1931 film Frankenstein, the 1935 sequel Bride of Frankenstein, and the 1939 sequel Son of Frankenstein.