Background: In PREVENT, eculizumab was associated with a significant reduction in relapse risk versus placebo and was well tolerated. In total, 46 patients (26/96 in the eculizumab arm, 20/47 in the placebo arm) were previously treated with the monoclonal antibody rituximab. Objectives: To describe the efficacy and safety of eculizumab in patients in the PREVENT trial (NCT01892345) who had previously received rituximab. Methods: Adults with aquaporin-4 immunoglobulin G-positive neuromyelitis optica spectrum disorder received eculizumab (maintenance dose, 1200 mg/2 weeks) or placebo with/without concomitant immunosuppressive treatment (except rituximab/mitoxantrone). A post hoc descriptive analysis was performed using data from patients with any prior rituximab treatment (within the previous year only for review of adverse events [AEs]) recorded more than 3 months before randomization. Results: Baseline characteristics of the prior-rituximab subgroup were similar to those of the total PREVENT population; however, the subgroup included a lower proportion of Asian patients (10.9% vs 36.4% in total PREVENT) and greater representation from the Americas (58.7% vs 30.8%). In the subgroup, median times from last dose of rituximab to meningococcal vaccination and to first dose of study treatment were 31.7 and 38.7 weeks, respectively. Adjudicated relapses occurred in 1/26 patients (3.8%) and 7/20 patients (35.0%) in the eculizumab and placebo arms (hazard ratio: 0.093; 95% confidence interval: 0.0110.755; p = 0.0055), respectively. Rates of AEs for eculizumab and placebo were 1025.8 and 1029.1 events/100 patient-years (100% of patients), respectively, and rates of serious AEs were 46.9 and 66.0 events/100 patient-years (38.9% and 47.1% of patients), respectively. Serious infections/infestations were recorded in 2/18 patients (11.1%) and 2/17 patients (11.8%) in the eculizumab and placebo arms, respectively. Conclusions: In patients in PREVENT who had previously received rituximab, the risk of adjudicated relapse was significantly lower with eculizumab than with placebo. Rates of serious infections were similarly low with eculizumab and placebo.