A parosteal (juxtacortical) osteosarcoma case was diagnosed on the lateral aspect of the left tarsal joint of an 11-year-old male Siamese-crossbred cat. A radiolucent line of demarcation between the tumor and the underlying cortical bone was present. The immobile mass was of soft to firm consistency with cystic areas on the cut surface. Histologically, the tumor tissue consisted generally of large, ovoid or fusiform cells with vesicular nuclei and partially of epithelioid like mesenchymal elements and abundant osteoclastic giant cells. Cellular pleomorphism and mitotic activity were found. The tumor cells were observed to have produced partially hyalinized collagen like osteoid matrix and constituted few crude osteochondroid areas. Partial aneurismal cysts like vascular structures in association with numerous giant cells resembling osteoclasts were present in the tumoral mass with marked necrosis. Based on the radiological and histomorphological features, the tumor with numerous osteoclasts and aneurysmal cystic changes was classified as parosteal osteoclastic osteosarcoma.