Extra-skeletal osteosarcoma is a malignant mesenchymal neoplasm that originates from soft tissue. This tumor occurs spontaneously in dogs, but there are few cases described in the literature. The objective of this study was to describe the epidemiological and pathologic features of extra-skeletal osteosarcoma in 36 dogs. Cases diagnosed as osteosarcoma affecting dogs (n=216), during 2006 to 2016 were reviewed. Of these, 36 cases were diagnosed as extra-skeletal osteosarcoma. The mean age of affected dogs was 10.4 years, and the mean weight was 19.5kg. No breed predisposition was observed. The most frequently affected sites were the mammary glands (80.6%; 29/36), subcutaneous tissue (5.6%; 2/36), liver (5.6%; 2/36), spleen (2.8%; 1/36), omentum (2.8%; 1/36), and kidney (2.8%; 1/36). Histological examination revealed that 61.1% (22/36) of the tumors were osteoblastic, 33.3% (12/36) chondroblastic, 2.8% (1/36) fibroblastic, and 2.8% (1/36) were giant cell-rich osteosarcomas. Histological grade II and III were the most frequent. Lymph node metastases were observed in 21.4% (3/14) of the cases of mammary gland osteosarcoma. Distant metastases were observed in 23.1% (3/13) of cases by diagnostic imaging and in 71.4% (5/7) during autopsy. Metastases were observed in the lungs (57.1%; 4/7), liver (14.3%; 1/7), spleen (14.3%; 1/7), and in multiple sites (14.3%; 1/7). Pulmonary metastases were more frequently observed in cases of mammary gland osteosarcoma.