Harrisia is a genus of cacti native to South America and the Caribbean. Herbarium specimens, field studies, and cultivated plants were used to revise the genus. Harrisia caymanensis is described from the Cayman Islands. Typifications of 10 names of Harrisia are designated here. The type of H. taetra is clarified. Four names are newly synonymized. An identification key is provided for the 18 recognized species.