A juvenile female California sea lion (Zalophus californianus) presented with a firm subcutaneous mass in the right caudoventral pelvic region, measuring approximately 15 × 15 cm, and edema in the right hind flipper. The initial diagnostic workup included an ultrasound, blood analysis, and fine-needle aspirate of the mass under manual restraint. While the blood analysis was within normal limits, the ultrasound revealed a large discrete hyperechoic homogeneous mass with anastomosing hypoechoic fluid pockets that, when aspirated, was suggestive of a reactive lymph node. Short-term empiric antibiotic therapy prescribed to address possible lymphadenitis yielded no response, and edema of the right hind flipper and perianal region worsened. The sea lion was anesthetized to facilitate a more thorough exam, repeat blood analysis, obtain survey radiographs, evaluate with ultrasound, and perform biopsy of mass. The thoracic radiographs were normal; however, the large radiopaque mass was observed cranial to the pelvis on lateral abdominal view and appeared solitary on ultrasound. An exploratory surgery was performed to completely excise the mass. On histopathologic review, the mass demonstrated diffuse fibrous hyperplasia of lobular ducts of the mammary gland and mild mastitis, characterizing it as a mammary fibroadenoma. The surgical wound healed within 6 wk, and there was no recurrence of mammary hyperplasia.