Mermen are mythical male legendary creatures who are human from the waist up and fish-like from the waist down, whose consorts were their female counterparts, the more commonly known mermaids. In Greek mythology, mermen were often illustrated to have green seaweed-like hair and beards.

The actions and behavior of mermen can vary wildly depending on the source and time period of the stories. They have been said to sink ships by summoning great storms, but also said to be wise teachers, according to earlier mythology.

The most well-known merman was probably Triton, son of Poseidon and Amphitrite. Although Amphitrite gave birth to a merman, neither Poseidon nor Amphitrite were merfolk, although both were able to live under water as easily as on land. Triton was also known as the Trumpeter of the Sea for his usage of a conch shell.

Other noteworthy mermen were the Babylonian Oannes and Ea and the Sumerian Enki.

In UFOlogy, the alien race called Nommos are mermen from Sirius who, according to some texts, visited ancient Egypt.