A 2‐year‐old, male German shepherd dog presented with a chronic history of hindlimb gait abnormalities, urinary and faecal incontinence. Magnetic resonance imaging of the area revealed a diffuse subarachnoid diverticulum (from T6 to L2), and a novel subarachnoid‐peritoneal shunting technique was considered. Surgery involved a T11–T13 dorso‐lateral hemilaminectomy and placement of a lumbar catheter into the diverticulum via durectomy. The catheter was then inserted into the peritoneal cavity via a left lateral incision to the abdomen. The dog recovered well from the surgical procedure and showed a resolution of its clinical signs at the follow‐up appointment (2 weeks after surgery). The patient represented 7 months later for recurrence of urinary and faecal incontinence and mild pelvic limb ataxia. Repeated diagnostic imaging revealed kinking of the lumbar catheter. Revision surgery was performed and the patient showed a resolution of the pelvic limb ataxia the following day. Patient lost to long‐term follow‐up.