A petasos or petasus is a sunhat of Thessalian origin, worn by the ancient Greeks, often in combination with the chlamys cape. It was usually made of wool felt, leather or straw, with conical crown and a broad, floppy brim. The petasos had a chin strap, so that when not in use it could be hung down the back. The petasos worn by men had a rather low crown, while that worn by women had a tall one. 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 (Roman equivalent Mercury).
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.
The petasos is often misdescribed as a kausia.