Background: To evaluate the efficacy and safety of HLX01, a rituximab biosimilar, as combination therapy with methotrexate in Chinese patients with active rheumatoid arthritis who had inadequate responses to methotrexate. Methods: In this double-blind, placebo-controlled phase 3 trial, biologic-naive patients with moderate-to-severe active rheumatoid arthritis and inadequate responses to methotrexate were randomized 2:1 to receive 1000 mg HLX01 or placebo intravenously on days 1 and 15. On the first day of weeks 24 and 26, patients in both groups received 1000 mg HLX01 via intravenous infusion. The primary endpoint was the American College of Rheumatology (ACR) 20 response rate at week 24. Secondary endpoints including efficacy, safety, immunogenicity, pharmacokinetics and pharmacodynamics were assessed up to week 48. Results: Between 28 May 2018 and 11 September 2020, 275 patients were randomized to the HLX01 group (n = 183) or the placebo group (n = 92). At week 24, the proportion of patients achieving ACR20 response was significantly greater in the HLX01 group compared with the placebo group in the intention-to-treat population (60.7% vs 35.9%; P < 0.001) and per-protocol set (60.3% vs 37.1%; P < 0.001). Most secondary efficacy endpoints favoured HLX01 when assessed at weeks 12, 24, 36 and 48. Incidences of treatment-emergent adverse events were similar between groups. Infusion-related reactions occurred more frequently following the initial two doses of HLX01 than the subsequent doses. Conclusions: HLX01 plus methotrexate improved clinical outcomes compared with placebo in Chinese patients with rheumatoid arthritis who had inadequate responses to methotrexate. This treatment regimen was well tolerated, showing comparable safety profiles to placebo.