Endometrial cancer (EC) can coexist with gynaecological diseases including adenomyosis which can be defined as the location of endometrial cells within the myometrium. Our aim was to clarify the impact of adenomyosis on the clinical and survival outcomes of EC. We included the patients who underwent an operation for EC and had concurrent adenomyosis in this retrospective cohort study. All clinicodemographical and tumour characteristics with survival outcomes of the patients were analysed comparatively. A total of 763 patients who met the eligibility criteria were included. Of those, 591 patients in the non-adenomyosis group and 172 patients in the adenomyosis group were examined. Disease-specific survival (DSS) was significantly prolonged while progression-free survival (PFS) was not affected by the presence of adenomyosis (p = .004 vs. p = .161). However, adenomyosis did not remain as an independent prognostic factor for EC in multivariate analysis (p = .341). These patients with coexistent adenomyosis and EC have better clinicopathological characteristics and less advanced tumour. Although adenomyosis is associated with prolonged DSS, it has no prognostic importance for survival outcomes of the patients with EC.IMPACT STATEMENT What is already known on this subject? Endometrial cancer (EC) can coexist with other gynaecological diseases including uterine adenomyosis. Adenomyosis is typically diagnosed by the pathological evaluation of the uterus following hysterectomy, although diagnosis is possible with imaging methods However, the coexistence of adenomyosis and EC is controversial in the literature. What do the results of this study add? To the best of our knowledge, our study is the largest study performed at a single university hospital. All potential confounding factors including clinicodemographical characteristics of the patients, examination of histopathology slides by the experienced gynaecological pathologists, evaluation of all included factors that may affect the survival outcomes of EC by multivariate analysis were examined. Although adenomyosis is associated with prolonged disease-specific survival (DSS), it has no prognostic importance for survival outcomes of the patients with EC. What are the implications of these findings for clinical practice and/or further research? Women having coexistent adenomyosis and EC should be informed about the impact of adenomyosis on the survival outcomes of EC.