This case report describes the clinical, sonographic, computed tomographic and pathological findings in a 9-year-old goat with mediastinal lymphocytic thymoma. The goat was referred to the Department of Farm Animals because of weight loss and dyspnoea. The lead clinical findings were increased heart rate, increased respiratory rate and heart sounds heard only on the right side. Ultrasonographic examination revealed a massive amount of fluid and an echogenic corrugated mass ventral to the lungs in the thoracic cavity on the left side. Computed tomography showed that the mass was very large and diffusely mineralised. A tentative diagnosis of mediastinal neoplasia was made, and the goat was euthanized. Postmortem examination revealed a cauliflower-like, pedunculated tumour, which occupied the entire left thoracic cavity and displaced the left lung. Based on histological evaluation, the tumour was diagnosed as a lymphocytic thymoma.