Background: In PRIME (NCT03709316), niraparib significantly reduced the risk of disease progression or death versus placebo (PBO) (hazard ratio [HR], 0.45; 95% confidence interval [CI], 0.34–0.60) in Chinese patients (pts) with newly diagnosed, advanced ovarian cancer (OC), regardless of biomarker status. As response to chemotherapy is deemed to be associated with prognosis in OC, this subgroup analysis of the PRIME study aims to better understand niraparib treatment effect in pts based on response to first-line platinum-based chemotherapy (1L CT). Methods: This randomized, double-blind, PBO-controlled, phase 3 trial enrolled adults with newly diagnosed, stage III or IV OC who achieved a complete response (CR) or partial response (PR) to 1L CT and received primary or interval cytoreductive surgery, irrespective of residual disease status after surgery. Pts were randomized (2:1) to receive niraparib or PBO, whose starting doses were individualized based on baseline bodyweight and platelet count, with stratification according to status of germline BRCA mutations (yes or no), tumor homologous recombination status (deficient or proficient), receipt of neoadjuvant chemotherapy (yes or no), and clinical response to 1L CT (CR or PR). This prespecified exploratory analysis reports progression-free survival (PFS) and HRs based on clinical response to 1L CT. The data cut-off date was 30 September 2021. Results: Of the 384 pts randomized, 315 (82.0%) and 69 (18.0%) had a CR (212 niraparib, 103 PBO) or PR (43 niraparib, 26 PBO) to 1L CT, respectively. Baseline characteristics are presented in the Table. The overall PFS median follow-up was 27.5 months. Niraparib significantly extended PFS compared with PBO: the median PFS was 29.4 months for niraparib versus 8.3 months for PBO (HR=0.45; 95% CI, 0.32–0.61; P<0.001) in the CR group and was 19.3 months for niraparib versus 8.3 months for PBO (HR=0.45; 95% CI, 0.23–0.86; P=0.014) in the PR group. Conclusions: In pts with newly diagnosed advanced OC, PFS was substantially prolonged with niraparib versus PBO, regardless of response to chemotherapy and biomarker status. Moreover, pts who achieved a CR appeared to receive larger PFS benefit from niraparib than those with a PR. Clinical trial information: NCT03709316.