Complications of submucosal myomas during pregnancy are rare and can be divided into those occurring during pregnancy, at delivery, and in the puerperium. The authors present two similar cases of unusual complication of submucosal uterine myoma in the puerperium. The two patients were admitted to this department for protruding vaginal mass after full-term vaginal delivery. Each experienced sudden severe abdominal pain, and felt a large vaginal mass after defecation or coughing. Combining gynecological examination and MRI, the authors accurately diagnosed the cases as spontaneous prolapse of large pedunculated uterine submucosal myoma. After antiinfective therapy, the 45-year-old woman underwent total laparoscopic hysterectomy; the second case, a 31-year-old woman was treated by a conservative surgical approach, including partial vaginal myomectomy and elective hysteroscopic myomectomy. This is the first report describing spontaneous prolapse of pedunculated uterine submucosal myoma in the puerperium after full-term vaginal delivery, and MRI played a key role in the diagnosis and treatment.