Gaißau is a municipality in the district of Bregenz, in the Austrian state of Vorarlberg. It lies in Austria's westernmost region, where Vorarlberg borders both Germany and Switzerland.
The district of Bregenz takes in the area around the provincial capital of the same name, at the eastern end of Lake Constance. Gaißau is one of the smaller municipalities within that district.
As a municipality, Gaißau forms the lowest level of local government in Austria, with its own local administration.