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Background: This randomised, double-blind, placebo-controlled, parallel-group, 52-week Phase III study (MERIT; NCT04607005) assessed mepolizumab efficacy and safety in patients with chronic rhinosinusitis with nasal polyps (CRSwNP)/eosinophilic CRS (ECRS) in Japan, Russia, and China, for which data are limited., Methodology: Eligible patients (enrolled at 60 centres) had blood eosinophil count >2%, endoscopic bilateral NP score ≥5, nasal obstruction visual analogue scale (VAS) score >5, ≥2 sinonasal symptoms, and either previous sinus surgery or systemic corticosteroid use/intolerance. Patients were randomised (1:1) to receive mepolizumab 100 mg subcutaneously or placebo every 4 weeks, plus standard of care. Co-primary endpoints: change from baseline in total endoscopic NP score (ENPS) (Week 52) and nasal obstruction VAS score (Weeks 49-52). Post hoc analyses conducted in a modified intent-to-treat (mITT) population excluded patients from two study sites, related to Good Clinical Practice violations by the Site Management Organisation overseeing these sites. These were considered the primary efficacy analyses., Results: In the mITT population, mepolizumab (n=80) versus placebo (n=83) significantly improved nasal obstruction VAS score from baseline to Week 49-52 and was associated with a trend of total ENPS improvements at Week 52. Mepolizumab/placebo on-treatment adverse events (AEs) occurred in 68/84 and 65/85 patients in the safety population (treatment-related AEs: 2/84 and 5/85, respectively), and on-treatment serious AEs in 0/84 and 4/85 patients, respectively (no fatalities reported)., Conclusions: Mepolizumab was effective and well-tolerated in patients with CRSwNP/ECRS from Japan, Russia, and China.