Objective: The benefit of adjuvant chemotherapy for FIGO stage I, high-grade serous ovarian cancer (HGSOC) after optimal staging is a matter of debate. We investigated the effect of adjuvant chemotherapy on recurrence-free survival (RFS) and overall survival (OS) in a population-based cohort study. Methods: All patients diagnosed in the Netherlands between 2002 and 2014 with FIGO stage I HGSOC who underwent surgical staging were included. Data on clinical characteristics, histopathology, completeness of staging and survival were collected from the Netherlands Cancer Registry and Dutch Pathology Registry. Recurrence data was collected from hospital files. We used Kaplan-Meier methods to estimate RFS and OS and Cox-proportional hazard analyses to control for differences in baseline characteristics between patients who did or did not receive chemotherapy. Results: We identified 223 patients who underwent optimal staging procedures including lymph node sampling. Events of disease recurrence occurred in 21 of the 101 patients (21%) who received adjuvant chemotherapy and in 46 of the 122 patients (38%) who did not (multivariable hazard ratio (HR), 0.37; 95%CI 0.22–0.64; p < 0.01). Five-year RFS was 81% after staging plus chemotherapy and 59% after staging only. At a median follow-up of 105 months, 21 patients (21%) in the chemotherapy group and 38 patients (31%) in the no-chemotherapy group had died (multivariable HR 0.50; 95%CI 0.28–0.89; p = 0.02). Ten-year OS was 78% with chemotherapy and 62% without chemotherapy. Conclusions: Adjuvant chemotherapy improves long-term RFS and OS in patients with FIGO stage I HGSOC after optimal staging.