During a preventive medicine examination on a 13-yr-old intact female African spot-necked otter (Lutra maculicollis), radiographs were obtained and a cranial thoracic mass was noted. Cytology from an ultrasound-guided fine-needle aspirate was suggestive of a thymoma. Surgical removal was performed and this diagnosis was confirmed by histopathology. At a recheck examination 6 mo postsurgery, there was no evidence of recurrence. This case highlights the value of a comprehensive preventive medicine program that resulted in the early detection of a subclinical thymoma and its successful removal.