A case of spontaneosly ruptured splenic hemangiosarcoma with abdominal dissemination in a 12-year-old male German shepherd dog is described. At necropsy, a ruptured tumor mass was seen in the ventral region of the visceral surface of the spleen, and disseminated tumor masses were seen throughout the peritoneal cavity. Histologically, in both the primary tumor mass and disseminated tumor masses, atypical endothelial cells lining vascular spaces as small clefts or as cavernous channels were observed. Some immunological properties of tumor cells were determined for endothelial cell markers (factor VII-related antigen=von Willebrand's factor [vWf] and CD31 antigen), mesenchymal tissue marker (vimentin) and pericyte-smooth muscle marker (α-smooth muscle actin [α-SMA]). Tumor cells were positive for CD31, vimentin and α-SMA, whereas they were negative for vWf. According to morphological and immunohistochemical results, this case was diagnosed as hemangiosarcoma arising from the spleen with abdominal dissemination.