<b><i>Background:</i></b> A better endometrial cancer (EC) prognosis in patients with coexistent adenomyosis has been hypothesized based on a different prevalence of favorable EC histological prognostic factors. However, pooled risk of EC unfavorable histological prognostic factors in patients with adenomyosis has never been calculated. <b><i>Objectives:</i></b> We aimed to assess the risk of EC unfavorable histological prognostic factors in patients with adenomyosis. <b><i>Methods:</i></b> All studies with data about histological prognostic factors of EC in patients with and without adenomyosis were included. Relative risk for each unfavorable histological prognostic factor of EC, such as nonendometrioid histotype, FIGO grade 3, FIGO stage II–IV, lymphovascular space invasion (LVSI), and deep myometrial invasion, was calculated in patients with adenomyosis compared to patients without adenomyosis. <b><i>Results:</i></b> Seven studies with 4,439 patients were included in the quantitative analysis. EC patients with adenomyosis showed a pooled RR of 0.77 (<i>p</i> = 0.05) for nonendometrioid histotype, 0.55 (<i>p</i> &#x3c; 0.00001) for FIGO grade 3, 0.60 (<i>p</i> = 0.005) for FIGO stage II–IV, 0.75 (<i>p</i> = 0.004) for LVSI, and 0.65 (<i>p</i> = 0.001) for deep myometrial invasion. <b><i>Conclusion:</i></b> EC patients with adenomyosis have a significantly decreased risk for unfavorable histological prognostic factors of EC compared to EC patients without adenomyosis. Such findings might explain the supposed better EC prognosis in patients with adenomyosis.