Introduction There is no prior case report of calculus within a female urethral diverticulum causing urinary retention. We present such a case successfully treated by transurethral lithotripsy. Case presentation A 34‐year‐old bedridden and uncommunicative woman with spinocerebellar degeneration presented with fever for 5 days. She was admitted to the hospital for a urinary tract infection with a 3‐cm calculus in the lower urinary tract. At the time of admission, acute urinary retention occurred. A bladder catheter was placed, and antibiotics were administered; both improved the urinary tract infection. Subsequently, transurethral lithotripsy was performed and revealed that the giant calculus was incarcerated within the urethral diverticulum. The bladder catheter was removed postoperatively, and urinary retention did not recur. No calculus reformation or urinary tract infections were observed for 6 months after discharge. Conclusion A giant calculus within a urethral diverticulum may cause acute urinary retention in an uncommunicative patient.