A 22-year-old captive male South American sea lion (Otaria flavescens) developed an undifferentiated carcinoma originating in the cardiac region of the stomach. Clinical symptoms included vomiting, anorexia and weight loss. Ultrasonography and endoscopy showed gastric wall thickness. At necropsy, the gastric wall had significant thickening around the cardiac region, and metastases were found in some organs. Histologically, samples from the stomach wall and metastases showed the same tumor tissue. Immunohistochemistry was positive for epithelium markers. Ductal growth, keratinocytes or signet ring cells were absent. The tumor was classified as an undifferentiated carcinoma using the World Health Organization's (WHO) guide to international classification of tumors in domestic animals. This is the first report of a primary gastric carcinoma in a pinniped.