Background The efficacy, safety, and pharmacokinetics of the combination of three direct-acting antiviral (DAA) agents (adafosbuvir [also known as AL-335], odalasvir, and simeprevir) were investigated in DAA treatment-naïve Japanese patients with genotype (GT)1 or GT2 chronic hepatitis C virus (HCV) infection, with or without compensated cirrhosis. Methods In this Phase IIa, open-label, multicenter study—OMEGA-3 (NCT02993250)—patients received JNJ-4178 (adafosbuvir 800 mg once daily [QD], odalasvir 25 mg QD, and simeprevir 75 mg QD) for 8 (non-cirrhotic patients; Cohort 1) or 12 (cirrhotic patients; Cohort 2) weeks. Patients were followed-up to 24 weeks following the end of treatment (EOT). The primary endpoint was safety, including adverse events (AEs). Results Overall, 33 patients were enrolled into Cohort 1 ( N = 22) or 2 ( N = 11) and received combined treatment with JNJ-4178. During the treatment and follow-up phases, a higher percentage of patients in Cohort 2 (81.8%) experienced AEs compared with Cohort 1 (68.2%), but the incidence of treatment-related AEs was similar. Most AEs were mild-to-moderate in severity and no patients discontinued due to an AE. There was one serious AE (cataract) in a patient in Cohort 2, which was not considered related to treatment. All patients achieved sustained virologic response 12 weeks after EOT (SVR12). No incidences of viral relapse were observed during follow-up. Conclusions In HCV GT1- and GT2-infected Japanese patients, treatment with JNJ-4178 was well tolerated and resulted in 100% of patients achieving SVR12. Electronic supplementary material The online version of this article (10.1007/s00535-020-01672-0) contains supplementary material, which is available to authorized users.