Background: Because of its low incidence, medical treatment of has not yet been well established although surgical excision is generally considered effective. We report the first case of primary bladder endometriosis successfully managed with a novel progestin dienogest. Case: A 39-year-old woman, nulligravida, presented with lower urinary tract symptoms, especially during menstruation. Cystoscopy, with subsequent cold cup biopsy, revealed a solitary submucosal mass (2 × 2 cm) in the bladder on the posterior wall; histopathology revealed the diagnosis of extraperitoneal endometriosis. MRI and laparoscopy confirmed no peritoneal endometriosis implants or adenomyosis. She was treated with oral 2 mg/day dienogest for six months. The measurable lesion exhibited a remarkable reduction in its size, accompanied with immediate relief of the lesion-related symptoms. At one year after medication cessation, she is well and symptom free. Conclusion: Dienogest may be a novel conservative alternative for bladder endometriosis, in particular for women who wish to avoid surgical intervention.