Background Bruton's tyrosine kinase (BTK) regulates the functions of B cells and myeloid cells that are implicated in the pathogenesis of multiple sclerosis. Evobrutinib is a selective oral BTK inhibitor that has been shown to inhibit B-cell activation both in vitro and in vivo. Methods In this double-blind, randomized, phase 2 trial, we assigned patients with relapsing multiple sclerosis to one of five groups: placebo, evobrutinib (at a dose of 25 mg once daily, 75 mg once daily, or 75 mg twice daily), or open-label dimethyl fumarate (DMF) as a reference. The primary end point was the total (cumulative) number of gadolinium-enhancing lesions identified on T-1-weighted magnetic resonance imaging at weeks 12, 16, 20, and 24. Key secondary end points included the annualized relapse rate and change from baseline in the score on the Expanded Disability Status Scale (EDSS). Results A total of 267 patients were randomly assigned to a trial group. The mean (+/- SD) total number of gadolinium-enhancing lesions during weeks 12 through 24 was 3.85 +/- 5.44 in the placebo group, 4.06 +/- 8.02 in the evobrutinib 25-mg group, 1.69 +/- 4.69 in the evobrutinib 75-mg once-daily group, 1.15 +/- 3.70 in the evobrutinib 75-mg twice-daily group, and 4.78 +/- 22.05 in the DMF group. The baseline adjusted rate ratios for the total number of lesions over time as compared with placebo were 1.45 in the evobrutinib 25-mg group (P=0.32), 0.30 in the evobrutinib 75-mg once-daily group (P=0.005), and 0.44 in the evobrutinib 75-mg twice-daily group (P=0.06). The unadjusted annualized relapse rate at week 24 was 0.37 in the placebo group, 0.57 in the evobrutinib 25-mg group, 0.13 in the evobrutinib 75-mg once-daily group, 0.08 in the evobrutinib 75-mg twice-daily group, and 0.20 in the DMF group. There was no significant effect of trial group on the change from baseline in the EDSS score. Elevations in liver aminotransferase values were observed with evobrutinib. Conclusions Patients with relapsing multiple sclerosis who received 75 mg of evobrutinib once daily had significantly fewer enhancing lesions during weeks 12 through 24 than those who received placebo. There was no significant difference with placebo for either the 25-mg once-daily or 75-mg twice-daily dose of evobrutinib, nor in the annualized relapse rate or disability progression at any dose. Longer and larger trials are required to determine the effect and risks of evobrutinib in patients with multiple sclerosis. (Funded by EMD Serono; ClinicalTrials.gov number, .) In a randomized trial, patients who received evobrutinib, an inhibitor of Bruton's tyrosine kinase, at 75 mg daily had significantly fewer enhancing lesions on MRI during weeks 12 through 24 than those who received placebo. However, there was no significant between-group difference for either a lower or a higher dose of evobrutinib, or in the annualized relapse rate or disability progression at any dose.