A 12-year-old female with primary amenorrhea presented to the emergency department with episodic low back pain, suprapubic discomfort, and acute urinary retention. A protruding purple mass was noted at the vaginal introitus consistent with an imperforate hymen. Point-of-care bedside transabdominal ultrasonography revealed a distended uterus containing hypoechoic material. The patient underwent formal pelvic ultrasonography, which revealed a markedly enlarged uterus containing a large number of blood products, thinned myometrium, and a distended vaginal canal consistent with hematometrocolpos secondary to imperforate hymen. Imperforate hymen is a rare congenital anomaly of the female urogenital tract, in which the hymen obstructs the vaginal opening causing a vaginal outlet obstruction. Vaginal outlet obstruction secondary to imperforate hymen may lead to retrograde menstruation with a collection of blood within the uterus and vagina, which is termed hematometrocolpos. Treatment is based on identifying and treating the underlying imperforate hymen with surgical approaches. The growing use of bedside ultrasonography allows the clinician to rapidly and accurately diagnose hematometrocolpos. The use of point-of-care bedside ultrasonography can serve as an essential tool as delayed diagnosis and treatment of this rare condition are associated with significant morbidity and lifelong infertility.