Acquired bladder diverticula due to bladder outlet obstruction are not uncommon in the adult male population. Congenital diverticula originate adjacent to the trigone and are rarely diagnosed in adults. We report an unusual case of a diverticulum arising adjacent to an ectopic ureter located on the left lateral wall near the dome of the bladder. Although the diverticulum appeared to be congenital, its large size was likely a result of high pressure voiding. The patient underwent a transurethral resection of the prostate to reduce his bladder outlet obstruction, and subsequently underwent an open diverticulectomy.