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Malignant transformation of endometriosis has been extensively described in the literature. However, extragonadal endometrioid adenocarcinoma, either de novo or arising from malignant transformation of endometriosis, is rare. The present case report describes a patient with endometrioid adenocarcinoma on the serous surface of the small intestine. A 25- year-old female with no history of endometriosis was referred to our hospital with an intrapelvic tumor. An internal examination, ultrasound, and magnetic resonance imaging revealed a round mass approximately 80 mm in diameter; however, identification of the affected organ was difficult. Because we could not rule out malignancy based on the non-specific radiologic findings, laparotomy was performed. A mass with ileal adhesions was detected intraoperatively. In addition, the uterus and bilateral adnexa appeared normal. The tumor was resected with part of the ileum. Histopathology confirmed a diagnosis of endometrioid adenocarcinoma originating from the serous surface of the small intestine.