Introduction Patients with diffuse large B-cell lymphoma (DLBCL) have limited access to rituximab. IBI301 is a recombinant chimeric murine/human anti-CD20 monoclonal antibody and is a candidate biosimilar to rituximab. This study aimed to assess the therapeutic equivalence of IBI301 and rituximab in previously untreated patients with diffuse large B-cell lymphoma (DLBCL). Methods This multicenter, randomized, double-blind, parallel-group, phase 3 trial compared IBI301 and rituximab, both plus the chemotherapy of doxorubicin, cyclophosphamide, vindesine, and prednisone (CHOP), was conducted in 68 centers across China. Eligible patients with untreated CD20 positive (CD20(+)) DLBCL randomly received IBI301 (375 mg/m(2)) plus the standard CHOP or rituximab (375 mg/m(2)) plus the standard CHOP for six cycles of a 21-day cycle. The primary end point was the overall remission rate (ORR). Efficacy equivalence was defined if 95% CIs for the ORR difference between the two groups were within a +/- 12.0% margin. Results Between August 22, 2016, and September 5, 2018, 419 patients were randomly allocated into the IBI301 group (N = 209) and rituximab group (N = 210). In the full analysis set, the ORR was 89.9% and 93.8% in the IBI301 and rituximab groups, respectively, and the ORR difference was -3.9% (95% CI - 9.1%-1.3%), falling within a +/- 12.0% margin. The occurrences of treatment-emergent adverse events (TEAEs) (100% vs. 99.0%) and AEs of grade >= 3 (87.1% vs. 83.3%) were similar in the two groups (P > 0.05). Conclusions IBI301 had a non-inferiority efficacy and a comparable safety compared with rituximab. IBI301 plus CHOP could be suggested as a candidate treatment regimen for untreated patients with CD20(+) DLBCL.