A boater (also straw boater, basher, skimmer, cady, katie, canotier, somer, sennit hat, or in Japan, can-can hat) is a kind of men's formal summer hat.

It is normally made of stiff sennit straw and has a stiff flat crown and brim, typically with a solid or striped grosgrain ribbon around the crown. Boaters were popular as casual summer headgear in the late 19th century and early 20th century, especially for boating or sailing, hence the name. They were supposedly worn by FBI agents as a sort of unofficial uniform in the pre-war years. It was also worn by women, often with hatpins to keep it in place. Nowadays they are rarely seen except at sailing or rowing events, period theatrical and musical performances (e.g. barbershop music) or as part of old-fashioned school uniforms. Since 1952, the straw boater hat has been part of the uniform of the Princeton University Band, notably featured on the cover of Sports Illustrated Magazine in October, 1955. Recently, soft, thin straw hats with the approximate shape of a boater have been in fashion among women.