The accessory cavitated uterine mass (ACUM) is a rare obstructive malformation of the female genital tract. It is characterized by a cavitated mass with an inner lining of the endometrium filled with accumulated blood, located under the attachment of the round ligament to the uterus. The patient’s uterine cavity, fallopian tubes, and ovaries are normal. Patients with ACUM have clinical manifestations of progressive dysmenorrhea and can be diagnosed with the help of pelvic MRI and transvaginal ultrasonography. Small foci of adenomyosis could be found in the myometrium adjacent to the ACUM, but not in the rest of the uterus. Complete resection of the mass can fully relieve dysmenorrhea.