BACKGROUND: Despite the availability of several biologics for ulcerative colitis (UC), there remains a critical need to identify first-line treatment biologics. The superiority of infliximab (IFX) over vedolizumab (VED) and ustekinumab (UST) was evaluated as initial UC treatments in patients with biologic-naive UC. METHODS: This multicenter, randomized control trial was conducted across 20 Japanese medical institutions. An independent center randomly allocated patients with UC (Mayo score >/= 6) who had not previously used biologics to three treatment groups (IFX, VED, UST). The primary endpoint was the clinical remission (CR) rate at week 12, with other endpoints including the treatment continuation rate at week 26 and adverse events (AEs). RESULTS: From May 2021 to June 2023, 107 cases were registered, including 104 for safety and 97 for efficacy evaluation. CR rate at week 12 was 36.4% (95%CI:20.4-54.9), 32.4% (95%CI:17.4-50.5) and 43.3% (95%CI:25.5-62.6) in IFX, VED, and UST group, respectively. Continuation rates at week 26 were 50.0%(IFX), 58.3% (VED), and 82.4% (UST). AEs related to study medication were 14.7% (IFX), 16.7% (VED), and 5.9% (UST). Predictors for CR at week 12 were thiopurine use in IFX (p = 0.04), lower baseline Mayo score (p = 0.007), and lower Patient report outcome 2 (p = 0.003) at week 2 in VED. CONCLUSION: Due to small sample size, it is challenging to make conclusions for main endpoints from this study while our study suggested that use of thiopurines in IFX group and lower activity at enrollment in VED group may enhance treatment efficacy. (jRCT1031200329; available at https://jrct.niph.go.jp/ ).