A mermaid (from the Middle English mere in the obsolete sense 'sea' + maid(en) is a legendary aquatic creature with the head and torso of human female and the tail of a fish. The male version of a mermaid is called a merman; the gender-neutral collective noun is merfolk. Various cultures throughout the world have similar figures.

In some ancient cultures Mermaids were regarded as semi-divine aspects of the Goddess, connected to the sea from which life arises and honoured in seaside temples. The earliest Mermaid story comes from Assyria around 1000 BCE. Atargatis, an Assyrian priestess, jumpred into the sea to wash away the shame of an unwanted pregnancy and emerged as a fishtailed goddess. In the 2nd century BCE, the Greek historian Lucian reported that the statue of the Great Goddess at the temple of Hieropolis (which is now modern Turkey) had a fishtail instead of legs. In Greece, Aphrodite, the goddess of love, as born from the sea foam and rode to land on a half-scallop shell.

In the 1st century CE, Pliny wrote convincingly of the existence of Mermaids, but said that their bodies were 'rough and scaled all over'. But by the 5th century CE, the bestiary Physiologus described Mermaids in terms that accord fully with their contemporary image. Mermaids are 'wonderfully shaped as a maid from the navel up and fish from the navel down'.

Mermaids were often sighted by seamen during the middle ages, Christopher Columbus among them, who reported seeing three Mermaids on his first voyage to the Americas in 1493. Mermaids figured prominetly in sailors' lore, because of such travellers' tales. The most common story was that Mermaids were incredibly skilled at seducing lonely sailors and dragging them down to their underwater kingdom. It was also believed that they could cause storms and shipwrecks. These beliefs were reinforced by the medieval Church, to which Mermaids were an emblem of vanity, lust and the spiritual perils of women and of sexuality.

The Sirens of Greek mythology are sometimes portrayed in later folklore as mermaids; in fact in some languages the name sirena is used interchangeably for both creatures. Other related types of mythical or legendary creature are water fairies (e.g. various water nymphs) and selkies.