Introduction. Acute urinary retention in a child is rare. Haematocolpos can cause a mechanical obstruction, resulting in acute urinary retention. Case Report. A 12-year-old girl presented to the surgical department with a one-day history of acute urinary retention and suprapubic tenderness. She had not started menses but had described period-like pains every month for the past six months. On examination, she had a palpable bladder with over 500 mls of residual urine and a bluish-grey bulge posterior to her urethral meatus. An US scan showed a large mass posterior to her bladder resembling a haematocolpos, and this was confirmed with an MRI scan. She was catheterised and eventually underwent a hymenectomy using a cruciate incision. She made a good recovery postoperatively. Conclusion. In the case of a peripubertal female presenting with acute urinary retention, haematocolpos should be considered as a diagnosis.