Objective: To examine the association of coexistence of adenomyosis and endometrial carcinoma on tumor characteristics and survival outcome of patients. Methods: Clinical and pathological data were retrospectively reviewed from 1584 patients who underwent surgical treatment of endometrial carcinoma. Statistical analyses were performed to evaluate associations of the presence or absence of adenomyosis with demographics, clinical parameters, histopathological factors, and survival outcomes. Results: Adenomyosis was found in 150/1584 patients, and was significantly associated with premenopausal status (46% vs. 35.15%, P = 0.008), younger age (60.67% vs. 41.92% < 55 years old, P < 0.001), lower positive p53 expression (53.36% vs. 63.32%, P = 0.034), earlier disease stage (I-II) (92.67% vs. 85.56%, P = 0.016), lower grade of the tumors (1-2) (91.33% vs. 84.52%, P = 0.025), lower likelihood of outer-half myometrial invasion (10% vs. 22.25%, P < 0.001), and absence of pelvic lymph node metastasis (97.04% vs. 92.09%, P = 0.037). The presence of adenomyosis was also associated with better survival outcomes, with a higher 5-year survival rate (92.1% vs. 84.1%, P = 0.045). In multivariate analysis, age, BMI, stage/grade of tumors, and myometrial invasion were independent prognostic factors associated with survival outcomes. Conclusion: The presence of adenomyosis was associated with less aggressive behavior of endometrial cancer and is a protective factor associated with better outcomes of patients.