Chondrosarcoma was diagnosed in an aged female California sea lion. The animal presented initially with a distended abdomen. Abdominocentesis yielded a cloudy fluid with a total protein of 4.8 gm/100 ml, containing a few white blood cells and no bacteria. Ultrasonography and radiography identified multiple masses of various sizes throughout the abdominal cavity. The animal continued to eat well and appeared normal for four months at which time she was examined for sporadic inappetence and discomfort when out of the water. Radiographically, the masses had increased both in size and number. At necropsy, the abdomen contained approximately eight liters of fluid and multiple white solid and fluid filled masses, 0.5 to 35 centimeters in diameter, attached throughout the mesentery and floating free in the abdominal cavity. The largest mass, weighing 23 kilograms, was attached to the omentum and greater curvature of the stomach. Histopathologic diagnosis was chondrosarcoma, but a primary skeletal site could not be identified. Chondrosarcoma is an uncommon malignant neoplasm that has been reported in all domestic species of animals, especially sheep. The extraskeletal form of chondrosarcoma is extremely rare and to our knowledge, this is the first reported case in a marine mammal.