A petticoat is an article of clothing for women; specifically an undergarment to be worn under a skirt (also known as underskirt) or dress. The petticoat is a separate garment hanging from the waist (unlike the chemise). The practice of wearing petticoats was well established by 1585. By the middle of the 20th century, the petticoat was rare, having been commonly replaced by modern undergarments.

Petticoats were worn throughout history by women who wanted to have the currently fashionable shape created by their clothing. The petticoat(s), if sufficiently full or stiff, would hold the overskirt out in a pleasingly domed shape and give the impression of a smaller waist than the wearer actually had. It would also complement the desired large bust. The petticoat was not worn to hide the legs, as twentieth century commentators later claimed; it actually enhanced the stylish figure in the centuries before female attractiveness was defined almost wholly by how much naked leg was showing, as has been the case since 1960. Petticoats were revived by Christian Dior in his New Look of 1947 and remained extremely popular during the 1950s, especially with teenage girls.