A 12‐year‐old entire male cocker spaniel presented for re‐evaluation of mitral valve disease. Abdominal palpation revealed a large mass caudally. Abdominal ultrasonography identified a large, heterogeneous, mixed echogenicity mass adjacent to the bladder and prostate. A CT scan of the abdomen confirmed that the mass originated from the cranial aspect of the right prostatic lobe. The mass was of soft tissue attenuation, homogeneous, well circumscribed and displayed mild homogeneous contrast enhancement. There was overlap between the radiologic appearances of prostatic leiomyoma and prostatic carcinoma. Histopathological examination confirmed a prostatic leiomyoma. The patient did not display any clinical signs associated with this tumour and was still alive and asymptomatic three months after diagnosis. Prostatic leiomyomas are very rare in dogs, with only four cases identified within the literature over the previous 40 years; leiomyoma should be included in the differential diagnoses for an atypical prostatic mass in dogs.