A petasos or petasus (Greek: πέτασος) is a sun hat of Thessalian origin worn by ancient Greeks, Thracians and Etruscans, often in combination with the chlamys cape. It was usually made of wool felt, leather or straw, with a broad, floppy brim. It was worn primarily by farmers and travelers and was considered characteristic of rural people. As a winged hat, it became the symbol of Hermes, the Greek mythological messenger god.
A type of metal helmet worn by Athenian cavalry was made in the shape of a petasos. Some examples have holes around the outer edge of the brim, presumably so a fabric cover could be attached. These are known from reliefs and vase paintings, with at least one archaeological example found in an Athenian tomb.