A 12-year-old Domestic Shorthair cat with a soft, fluctuant, subcutaneous mass, approximately 5 cm in diameter on the posterior aspect of the left tarsus was diagnosed as having protothecosis. Cultures, histopathology, and fluorescent antibody reagents were used to identify Prototheca wickerhamii as the etiologic agent. Protothecosis has not previously been recorded in cats.