Rectal diverticulum is a rare condition in dogs characterized by formation of a pouch or sac due to hernial protrusion of the mucous membranes through a defect in the muscular coat of the rectum. A 12-year-old male terrier dog was admitted with a history of a left perineal swelling, dyschezia and tenesmus during the last five months. Digital rectal examination identified a weakness in the left pelvic diaphragm and feces-filled sac within the lateral wall of the rectum. Positive contrast radiography showed a marked solitary diverticulum (3.5 × 4 × 4.5 cm) with wide-orifice neck arising from the left rectal wall. Using a lateral approach, a large rectal diverticulum was found and diverticulectomy following standard herniorrhaphy was performed. The dog recovered uneventfully with no signs of dyschezia during the next three years. Diverticulectomy by lateral approach and perineal herniorrhaphy produced excellent results.