A seventy-three-year-old patient's vaginal examination revealed a semi-mobile cystic structure visualised at the middle third portion of the posterior vaginal wall. The cyst was excised and the cyst wall was sent for pathological examination. The histopathological examination of the cyst with CD10 staining and tests for nuclear estrogen receptor positivity and nuclear progesteron receptor positivity revealed that the cyst wall contained endometrial stromal cells, endometrial surface epithelial cells, and hemosiderin-laden macrophages. Endometriosis externa was diagnosed based on the above findings. The case we report here, of an isolated vaginal endometriotic cyst, is unique because of its presentation in a postmenopausal woman without any history of infertility, any gynecologic surgery, or hormone replacement therapy. The reasonable explanation for this case might include certain mechanisms such as the self-sustaining existence of endometriotic cells that were implanted during the premenopausal period to the vagina and that maintain local estrogen production. No single hypothesis can explain the pathophysiology of endometriosis, given all of its various forms and presentations.