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RATIONALE The extrauterine adenomyoma is rare and it is extremely rare outside the pelvic cavity. Herein, we reported the first case of a single extrauterine adenomyoma occurring in the lesser omentum. PATIENTS CONCERNS This case involved a 55-year-old woman who had undergone subtotal gastrectomy and omentectomy for gastric carcinoma. During postoperational pathological examination, 1 lymph node-like mass was coincidentally found in the lesser omentum. The patient had a history of hysterectomy for uterine leiomyoma 8 years ago. DIAGNOSES The resected 17 "lymph nodes" from the lesser omentum were routinely checked for possible metastasis of gastric carcinoma. One of lymph node-like mass was microscopically showed that it was composed of benign smooth muscle components, endometrial glands and stroma by HE staining. Therefore, adenomyoma was initially considered. INTERVENTIONS The lymph node-like mass was removed together with the lesser omentum during the subtotal gastrectomy and omentectomy for gastric carcinoma. No special intervention was performed for the adenomyoma. OUTCOMES Immunohistochemical staining confirmed that smooth muscle tissue was diffusely and strongly positive for Desmin, smooth muscle actin, estrogen receptor, and progesterone receptor, and negative for CD117, Dog-1, S100, and CD34. Endometrial glands and stroma were positive for estrogen receptor and progesterone receptor, and the endometrium interstitium was also positive for CD10. The final diagnosis of extrauterine adenomyoma occurring in the lesser omentum was established. LESSONS So far, to the best of our knowledge, total 53 cases of extrauterine adenomyoma have been reported in 45 English reports. The most common location for a single mass was pelvic cavity (37 cases), but rarely outside the pelvic cavity. This is the first case of a single extrauterine adenomyoma occurring in the lesser omentum.