A Santa suit is a costume worn by a person portraying Santa Claus.
The first appearance of a modern Santa Claus, complete with what we consider to be the Santa suit was in drawings by Thomas Nast. Nast's original drawings were of a small Santa who could slide down chimneys, but his later works made him full size. He was also the first to draw Santa wearing a red suit with fur lining, a nightcap, and a black belt with a large buckle.
Prior to Nast's work, Santa's outfit was tan in color, and it was he that changed it to red, although he also drew Santa in a green suit. This change is often mistakenly attributed to the work of Haddon Sundblom, who drew images of Santa in advertising for the Coca-Cola Company since 1931. Although Sundblom's work certainly changed the perception of Santa Claus, the red suit was shown on the covers of Harper's Weekly at least forty years before his work for the soda company was published. The Coca-Cola Company itself has attributed the red color of the suit to Nast's earlier work. Prior to the Coca-Cola advertising, the image of Santa was in a state of flux. He was portrayed in a variety of forms, including both the modern forms and in some cases as a gnome. It was Sundblom's work which standardised the form of Santa to the earlier Nast work, including the red suit outfit.