A boater (also straw boater, basher, skimmer, cady, katie, 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 as well, 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 uniform, such as at Harrow School. 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.

Inexpensive foam or plastic skimmers are sometimes seen at political rallies in the United States.

In Australia, New Zealand and South Africa the boater is still a common part of the school uniform in many boys schools, such as Shore School, Brisbane Boys' College, Knox Grammar School, Maritzburg College, South African College School, St John's College (Johannesburg, South Africa), Wynberg Boys' High School and numerous Christian Brothers schools (CCB).

The boater may also be seen worn by the 'carreiros' of Madeira, the drivers of the traditional wicker toboggans carrying visitors from the parish church at Monte (Funchal) down towards Funchal centre.

Being made of straw, the boater was and is generally regarded as a warm-weather hat. In the days when men all wore hats when out of doors, "Straw Hat Day", the day when men switched from wearing their winter hats to their summer hats, was seen as a sign of the beginning of summer. The exact date of Straw Hat Day might vary slightly from place to place. For example, in Philadelphia, it was May 15; at the University of Pennsylvania, it was the second Saturday in May.

The boater is a fairly formal hat, equivalent in formality to the Homburg, and so is correctly worn either in its original setting with a blazer, or in the same situations as a Homburg, such as a smart lounge suit, or with black tie. John Jacob Astor IV was known for wearing such hats.