Accessory and cavitated uterine mass is a rare developmental mullerian anomaly theorized to be related to gubernaculum dysfunction. It presents typically in young women as severe dysmenorrhea and chronic pelvic pain refractory to medical therapy. It is an accessory cavity lined by functional endometrium and surrounded by myometrium-like smooth muscle located in an otherwise normal uterus, typically located at the right anterior wall at the level of the round ligament attachment. Ultrasound, hysterosalpingography and magnetic resonance imaging are helpful tools to diagnose and distinguish this entity from a wide array of differential diagnoses. Surgical excision and histopathologic studies confirm the diagnosis and effectively relieves severe dysmenorrhea and chronic pelvic pain. This is a case of a 39 year old nulligravid who presented with severe dysmenorrhea initially diagnosed as rudimentary horn versus myoma. Excision revealed a cavitated mass containing chocolate-colored fluid within located at the right postero-fundal area. Histopathology revealed a diagnosis of accessory and cavitated uterine mass.