This case report describes the first documented occurrence of an ununited anconeal process (UAP) in a California sea lion (Zalophus californianus), expanding the species range of this orthopedic pathology beyond domestic canids. A 13-year-old female presented with chronic progressive right forelimb lameness. Diagnostic imaging revealed a large mineralized intra-articular bone fragment at the level of the elbow joint, accompanied by secondary osteoarthritic changes. The fragment was surgically excised via a lateral humero-ulnar arthrotomy. Histologic evaluation of the excised tissue confirmed an ununited ossification center with preserved trabecular architecture and no evidence of inflammation or neoplasia, supporting the diagnosis of UAP. Three-dimensional morphometric analysis demonstrated substantial remodeling of the ulna, humerus, and radius, with morphological deviation in pathological elements exceeding inter-individual variation observed in healthy conspecifics. Despite initial postoperative improvement, the animal succumbed to complications from an unrelated chronic diaphragmatic hernia, four weeks post-surgery. This report introduces UAP as a novel differential diagnosis for forelimb lameness in pinnipeds, and underscores the feasibility of surgical management. The findings provide crucial insights into the comparative pathology related to UAP and orthopedic surgical approaches in marine mammals. © 2025 by the authors.