Objective Prognostic factors associated with high-grade endometrial stromal sarcoma (HGESS) and undifferentiated uterine sarcoma (UUS) have not been distinctly determined due to the repetitive changes in the World Health Organization (WHO) classification. We aimed to compare clinicopathologic features and outcomes of patients with HGESS with those of patients with UUS. Methods A multi-institutional, retrospective, cohort study was conducted including 71 patients, who underwent surgery at 13 centers from 2008 to 2017. An experienced gynecopathologist from each institution re-evaluated the slides of their own cases according to the WHO2014 classification. Factors associated with refractory/progressive disease, recurrence or death were examined using logistic regression analyses. Kaplan-Meier method and log-rank test were used for survival comparisons. Results The median disease-free survival (DFS) for HGESS and UUS was 12 months and 6 months, respectively. While the median overall survival was not reached in HGESS group, it was 22 months in the UUS group. Kaplan-Meier analyses revealed that patients with UUS had a significantly poorer DFS than those with HGESS (p = 0.016), although OS did not differ between the groups (p = 0.135). Lymphovascular-space involvement (LVSI) was the sole significant factor associated with progression, recurrence or death for HGESS (Hazard ratio: 9.353, 95% confidence interval: 2.539-34.457, p = 0.001), whereas no significant independent factor was found for UUS. Conclusions UUS has a more aggressive behavior than HGESS. While no significant predictor of prognosis was found for UUS, LVSI is the sole independent prognostic factor for HGESS, with patients 9.3 times more likely to experience refractory/progressive disease, recurrence or death.