The clinical, radiographic, magnetic resonance imaging (MRI), surgical and pathological findings related to an osteochondral lesion of the sacrum in a mastiff dog are described. The dog showed chronic signs of pain in its pelvic limbs. Radiography revealed a triangular mineralised opacity at the craniodorsal aspect of the sacrum consistent with sacral osteochondrosis. A T2-weighted spin-echo MRI revealed dorsal and lateral compression of the cauda equina. The osteochondral fragment was removed via a dorsal laminectomy, and the clinical signs resolved. Histological abnormalities in the fragment were consistent with a diagnosis of osteochondrosis.