Purpose: Polycystic ovary syndrome (PCOS) is a risk factor for the development of endometrial cancer (EC). To the present authors’ knowledge, no study has clarified whether pre-existing PCOS is a prognostic factor for post-surgical EC recurrences. The aim of this study was to determine whether pre-existing PCOS is a risk factor for the recurrence of EC even after the surgical treatment of EC in premenopausal women. Materials and Methods: The authors performed a retrospective cohort study on premenopausal EC patients aged 50 years or younger who underwent surgery at this hospital between 2009 and 2013. The median follow-up period was 65.5 months. Results: Of 46 patients with EC, nine (19.6%) had PCOS. Four of the nine PCOS patients developed recurrence of EC, three of whom died of the disease, whereas only one of 37 patients who did not have PCOS developed EC recurrence (44.4% and 2.7%, respectively; p = 0.003). Univariate analysis showed that the progression-free and overall survival of the patients with pre-existing PCOS was worse than that of patients without pre-existing PCOS (p = 0.008 and p = 0.029, respectively). Multivariate analysis revealed that PCOS was a poor prognostic factor for progression-free survival and a marginal poor prognostic factor for overall survival (p = 0.011 and p = 0.061, respectively). Conclusions: Pre-existing PCOS was a risk factor for recurrence in premenopausal post-operative patients with EC aged 50 years or younger.