A mermaid is a mythical sea creature with the head and torso of a human woman and the tail of a fish. The male version is known as a merman, and the generic term is merfolk. The mermaid appears in folklore from all over the world, including Europe, the Middle East, the Caribbean, West and Central Africa, and Japan. As would be expected of any mythical figure with such a wide geographical range, the mermaid has many variations in appearance. The European Melusine, for example, often has a serpent's tail or two tails, sometimes in addition to wings.

Mermaids also vary in their reported behavior. Some are dangerous, such as the Greek Siren who sings enchanting songs from the shore in order to bewitch sailors or lure them to their death. Others are benevolent and may grant the wishes of those who see them. In Japan, eating the flesh of a mermaid is said to confer immortality. In the earliest known mermaid lore, heralding from first century BCE Assyria, the mermaid is a goddess who jumped into the sea in grief over the lover whom she accidentally killed.

The mermaid features extensively in the literature and art of many cultures and eras. She is the subject of one of Hans Christian Andersen's most well known fairy tales, The Little Mermaid, and the 1989 Disney movie based on it. Numerous films, cartoons, and television shows have also had the mermaid as their theme.

It is widely believed that tales of mermaids have their root in sightings of manatees, aquatic mammals that may resemble humans from a distance. The way that manatees carry their young, in particular, is said to resemble the way a human mother holds her child. Manatees often sun themselves on rocks, much as mermaids do in many accounts. Alternatively, seaweed may have given rise to mermaid legends, as it could have seemed to sailors like a mermaid's long hair floating underwater.