An 8-year old neutered male German Shepherd presented with vomiting, hindlimb weakness, and inappetence. A cranioventral mediastinal mass and an enlarged, gas-filled esophagus was found on thoracic radiography and computed tomography. Clinical and serological [findings] were consistent with the diagnosis of myasthenia gravis. The mass was surgically excised which resulted in gradual resolution of signs and decreased serum acetycholine receptor antibody titers. Histopathological evaluation of the mass confirmed a malignant thymoma. The dog's clinical signs recurred three months post-operatively and the dog was euthanized. Clincal signs, diagnostics, treatment of a dog with a thymoma and myasthenia gravis are described.