Müllerian mucinous borderline tumors (MMBTs) usually arise from the ovary. The present report is the first case of primary uterine MMBTs associated with adenomyosis. A 51-year-old woman was referred to our hospital for a complex cystic and solid 4×3 cm right adnexal mass. She had a history of a left ovarian endometriotic cyst and had undergone a left oophorectomy 2 yr prior. A laparotomy was performed, and the tumor was found to be originating in the posterior wall of the uterus. She underwent a total abdominal hysterectomy, right salpingo-oophorectomy, and left salpingectomy. Microscopically, the solid portion of the tumor contained papillary proliferations of glands, which were covered by a mucinous epithelium with mild to moderate nuclear atypia, accompanied by stromal infiltration of inflammatory cells. Islands of adenomyosis were also observed around the cyst. These pathologic findings were similar to the features of ovarian MMBT. We diagnosed this tumor as a uterine MMBT, probably arising from adenomyosis.