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Background: The PRIMA/ENGOT‐OV26/GOG‐3012 (NCT02655016) trial was amended to prospectively evaluate the safety and efficacy of an individualized starting dose (ISD) regimen of niraparib for first‐line maintenance therapy in patients with newly diagnosed advanced ovarian cancer. Methods: In the phase 3 PRIMA trial, patients with newly diagnosed advanced ovarian cancer with a complete/partial response to first‐line platinum‐based chemotherapy (N = 733) were initially treated with a fixed starting dose (FSD) regimen of 300 mg once daily. Subsequently, the protocol was amended so newly enrolled patients received an ISD: 200 mg once daily in patients with baseline body weight < 77 kg or baseline platelet count < 150,000/µL, and 300 mg once daily in all other patients. Efficacy and safety outcomes were assessed by starting dose. Results: Overall, 475 (64.8%) patients were assigned to an FSD (niraparib, n = 317; placebo, n = 158) and 258 (35.2%) were assigned to an ISD (niraparib, n = 170; placebo, n = 88). Efficacy in patients who received FSD or ISD was similar for the overall (FSD hazard ratio [HR], 0.59 [95% CI, 0.46–0.76] vs. ISD HR, 0.69 [95% CI, 0.48–0.98]) and the homologous recombination–deficient (FSD HR, 0.44 [95% CI, 0.30–0.64] vs. ISD HR, 0.39 [95% CI, 0.22–0.72]) populations. In patients with low body weight/platelet count, rates of grades ≥3 and 4 hematologic treatment‐emergent adverse events, dose interruptions, and dose reductions were lower for those who received ISD than for those who received FSD. Conclusions: In PRIMA, similar dose intensity, similar efficacy, and improved safety were observed with the ISD compared with the FSD regimen. Results from the phase 3 PRIMA trial of niraparib first‐line maintenance therapy that prospectively evaluated fixed versus individualized dosing support the selection of a niraparib starting dose based on an individual's body weight and platelet count. In prespecified analyses, patients who received the individualized starting dose of niraparib experienced similar progression‐free survival and improved safety outcomes compared with patients who received the fixed starting dose of niraparib. [ABSTRACT FROM AUTHOR]