This study presents a case of a canine thymolipoma, which is a rare, slow-growing, benign tumor of the thymus composed of mature adipose tissue and thymic tissue. A 9-year-old spayed, female miniature pinscher presented with chronic cough and dyspnoea. Radiology revealed pleural effusion and a mediastinal mass with a fatty appearance. The mass was attached to, and silhouetted, the adjacent pericardium. Microscopically, the mass was composed of adipose tissue with numerous cords and nests of thymic tissue without corticomedullary arrangement. To the authors' knowledge, this is the first description of this uncommon neoplasm in a dog. The gross and histological findings are similar to those described for thymolipomas in humans. The possible histogenesis for this neoplasia in a dog is also discussed.