A 4.5 year-old intact male mixed breed dog presented to the referring veterinarian on October 5, 2012 for evaluation of a soft, 3 x 5 cm subcutaneous mass on his left lateral thorax. A fine needle aspirate was not diagnostic, yielding only blood. On October 26, 2012, the referring veterinarian removed a 17 x 17 cm mass. Histopathological analysis revealed an incompletely excised subcutaneous hemangiosarcoma. The patient presented to the Oncology Service of the Cornell University Hospital for Animals on November 26, 2012 for examination and treatment. Upon presentation, he was bright, alert, and responsive with normal vital parameters. Aside from a healing surgical scar, physical examination findings were within normal limits. His bloodwork (complete blood count and serum chemistry), urinalysis, three view thoracic radiographs, and abdominal ultrasound were unremarkable. Using the case as a framework, this paper provides an overview of cutaneous hemangiosarcomas (dermal and hypodermal), including histological features, treatment options (including possible adverse effects), and prognostic indicators.