Adenomyosis is a common benign gynecological condition in women of reproductive age. While endometrial cancer co-existing with adenomyosis is common, endometrial carcinoma arising from endometrial cells within adenomyosis (EC-AIA) is rare. We present an unusual case of EC-AIA in a 62-year-old woman with no clinical symptoms. EC-AIA should be considered as a different diagnosis from concurrent endometrial cancer and adenomyosis. EC-AIA should also be considered in adenomyosis patients with intact endometrium and vaginal bleeding and in those with a history of adenomyosis at reproductive age regardless of the occurrence of vaginal bleeding. Periodic follow-up is recommended in women diagnosed with adenomyosis at reproductive age.