A 9 year-old, female spayed Rottweiler presented with a one and a half month history of growing masses on her right foreleg, dorsal and ventral neck, and left flank. Her only clinical sign was lameness on her right forelimb. CBC was normal, and blood chemistry had findings consistent with her previously diagnosed diabetes mellitus as well as mild hypoalbuminemia. Abdominal ultrasound and bone marrow aspiration were both normal. Thoracic radiographs showed three nodules in her lungs. Cytology done on fine needle aspirates of the masses, and biopsy of one of the masses were consistent with histiocytic sarcoma. She was initially treated with CCNU, and then switched to other chemotherapeutics after her disease progressed, but was eventually euthanized sixty-six days after diagnosis. This report gives an overview of the case as presented as well as a discussion of the presentation, diagnosis, treatment, and prognosis of histiocytic sarcoma in dogs.