A 6-year-old male castrated border collie presented with a 7-month history of tenesmus of up to 30 times per day with occasional frank blood noted on otherwise normal stool. Physical examination revealed an erythematous, swollen, and painful perianal area. Previous diagnostics and treatment plans had failed to resolve the clinical signs. An immune-mediated process was suspected and biopsies of the mucocutaneous junction involving both affected and normal tissues were submitted for histopathology. Mucocutaneous lupus erythematosus was diagnosed based on the clinical and histologic changes. The patient was started on a course of immune-modulating oral drugs including prednisone, niacinamide, and doxycycline. The clinical signs reached 80% resolution by the time of the suture removal and complete clinical resolution within 4 months of beginning the immune-modulating drugs.