Aim This multicenter investigation was performed to evaluate the adjuvant treatment options, prognostic factors, and patterns of recurrence in patients with grade 3 endometrioid endometrial cancer (G3-EEC). Materials and methods The medical reports of patients undergoing at least total hysterectomy and salpingo-oophorectomy for G3-EEC between 1996 and 2018 at 11 gynecological oncology centers were analyzed. Optimal surgery was defined as removal of all disease except for residual nodules with a maximum diameter <= 1 cm, as determined at completion of the primary operation. Adequate systematic lymphadenectomy was defined as the removal of at least 15 pelvic and at least 5 paraaortic LNs. Results The study population consists of 465 women with G3-EEC. The 5-year disease-free survival (DFS) and overall survival (OS) rates of the entire cohort are 50.3% and 57.6%, respectively. Adequate systematic lymphadenectomy was achieved in 429 (92.2%) patients. Optimal surgery was achieved in 135 (75.0%) patients in advanced stage. Inadequate lymphadenectomy (DFS; HR 3.4, 95% CI 3.0-5.6; P = 0.016-OS; HR 3.2, 95% CI 1.6-6.5; P = 0.019) was independent prognostic factors for 5-year DFS and OS. Conclusion Inadequate lymphadenectomy and LVSI were independent prognostic factors for worse DFS and OS in women with stage I-II G3-EEC. Adequate lymphadenectomy and optimal surgery were independent prognostic factors for better DFS and OS in women with stage III-IV G3-EEC.