A boater (also straw boater, basher, skimmer, The English Panama, cady, katie, canotier, somer, sennit hat, or in Japan, can-can hat, suruken) is a semi-formal summer hat for men, which was popularised in the late 19th century and early 20th century.

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 derived from the canotier straw hat worn traditionally by gondoliers in the city of Venice. The Venetian canotier has a ribbon that hangs freely off the back, and they are frequently edged with a matching color ribbon. Because of this, boaters were identified with boating or sailing, hence the name. Boaters were also identified more with sporting events and universities as well. They were also worn by women, often with hatpins to keep them in place. Nowadays they are rarely seen except at sailing or rowing events, period-related 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.

The boater is a semi-formal hat, equivalent in formality to the Homburg. As such, it 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. Actors Harold Lloyd and Maurice Chevalier were also famous for their trademark boater hats.

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

In the United Kingdom, Australia, and South Africa the boater is still a part of the school uniform in some very prestigious boys' schools, such as at Harrow School, Uppingham School, Shore School, Brisbane Boys' College, Knox Grammar School, Maritzburg College, Potchefstroom High School for Boys, South African College School, St John's College, Wynberg Boys' High School, Parktown Boys' High School and numerous Christian Brothers schools.

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 down towards Funchal centre.

Coco Chanel was fond of wearing boaters and made them fashionable among women during the early 20th century.

Boater hats of the late 19th century fin de siècle until World War I usually had wider brims than those afterwards. The narrower style that came into fashion following WWI was specifically known in Germany by the colloquial term Kreissäge ("circular saw"), whereas the official German term for it was (Florentiner) Strohhut ("(Florentine) straw hat").