Endometriosis-related neoplasms may arise within endometriotic cysts of organs of the female genital tract or other organ systems. Herein, we present a case of endometrioid adenocarcinoma arising within an endometriotic cyst in the mesosalpinx. A 27-year-old single female presented to the clinic with a history of right hypochondrial pain. Pelvic MRI revealed a bulky uterus with three intramural uterine fibroids and a complex cyst in the mesosalpinx. The excised cyst showed predominantly confluent growth of relatively well-defined glands resembling proliferative-phase endometrium in a background of endometriosis. The tumor was diagnosed as endometrioid adenocarcinoma within an endometriotic cyst, pathologic stage pT1a. She was referred to the oncology team, where a positron emission tomography scan showed unremarkable results. Although these neoplasms have been reported in various locations within the female genital tract, endometrioid adenocarcinoma arising from endometriosis in the mesosalpinx is seldom reported.