Accessory and cavitated uterine mass (ACUM) is a rare form of Mullerian anomaly that usually presents in young females with chronic cyclic pelvic pain and/or dysmenorrhea. This clinical entity is often underdiagnosed as it may be mistaken for other differential diagnoses, such as pedunculated myoma or adnexal lesions. Imaging modalities, including ultrasonography and magnetic resonance imaging (MRI), accompanied with relevant and suspicious clinical findings are important tools in making acorrect diagnosis. To date, surgical excision of the mass remains the mainstay of treatment,which provides significant symptom relief. In this study, we present a female adolescent with chronic pelvic pain since menarche who underwent laparotomy with the presumed diagnosis of a left-sided ovarian mass. Retrospective evaluation of pelvic MR images demonstrated that the lesion was in fact an ACUM, which was further confirmed by histopathological examination.