Background: No adjuvant treatment has been established for patients who remain at high risk for hepatocellular carcinoma recurrence after curative-intent resection or ablation. We aimed to assess the efficacy of adjuvant atezolizumab plus bevacizumab versus active surveillance in patients with high-risk hepatocellular carcinoma., Methods: In the global, open-label, phase 3 IMbrave050 study, adult patients with high-risk surgically resected or ablated hepatocellular carcinoma were recruited from 134 hospitals and medical centres in 26 countries in four WHO regions (European region, region of the Americas, South-East Asia region, and Western Pacific region). Patients were randomly assigned in a 1:1 ratio via an interactive voice-web response system using permuted blocks, using a block size of 4, to receive intravenous 1200 mg atezolizumab plus 15 mg/kg bevacizumab every 3 weeks for 17 cycles (12 months) or to active surveillance. The primary endpoint was recurrence-free survival by independent review facility assessment in the intention-to-treat population. This trial is registered with ClinicalTrials.gov, NCT04102098., Findings: The intention-to-treat population included 668 patients randomly assigned between Dec 31, 2019, and Nov 25, 2021, to either atezolizumab plus bevacizumab (n=334) or to active surveillance (n=334). At the prespecified interim analysis (Oct 21, 2022), median duration of follow-up was 17·4 months (IQR 13·9-22·1). Adjuvant atezolizumab plus bevacizumab was associated with significantly improved recurrence-free survival (median, not evaluable [NE]; [95% CI 22·1-NE]) compared with active surveillance (median, NE [21·4-NE]; hazard ratio, 0·72 [adjusted 95% CI 0·53-0·98]; p=0·012). Grade 3 or 4 adverse events occurred in 136 (41%) of 332 patients who received atezolizumab plus bevacizumab and 44 (13%) of 330 patients in the active surveillance group. Grade 5 adverse events occurred in six patients (2%, two of which were treatment related) in the atezolizumab plus bevacizumab group, and one patient (<1%) in the active surveillance group. Both atezolizumab and bevacizumab were discontinued because of adverse events in 29 patients (9%) who received atezolizumab plus bevacizumab., Interpretation: Among patients at high risk of hepatocellular carcinoma recurrence following curative-intent resection or ablation, recurrence-free survival was improved in those who received atezolizumab plus bevacizumab versus active surveillance. To our knowledge, IMbrave050 is the first phase 3 study of adjuvant treatment for hepatocellular carcinoma to report positive results. However, longer follow-up for both recurrence-free and overall survival is needed to assess the benefit-risk profile more fully., Funding: F Hoffmann-La Roche/Genentech., Competing Interests: Declaration of interests A-LC reports honoraria from BMS, Eisai, Ipsen, and Ono Pharmaceutical; consulting or advisory roles with Bayer Schering Pharma, BMS, Eisai, Exelixis, Ipsen, IQVIA, Merck Serono, Novartis, Nucleix, Omega Therapeutics, Ono Pharmaceutical, and Roche/Genentech; and participation on data monitoring or advisory boards for Abbisko Therapeutics. AOK reports honoraria from AstraZeneca, Bayer, BMS, Eisai, Exelixis, Merck, and Roche/Genentech; and travel expenses from Roche/Genentech. MK reports honoraria from Bayer, Chugai, Eisai, Eli Lilly, and Takeda; and grants or contracts from AbbVie, Chugai, EA Pharma, Eisai, GE Healthcare, Otsuka, and Taiho. HCL reports grants or contracts from AstraZeneca, MSD, and Roche. JH reports a leadership role in AstraZeneca; honoraria from Oncolys; consulting or advisory roles with AbbVie Korea; grants or contracts from Roche; and speaker's bureau from AstraZeneca, Gilead, Roche, and Yuhan Korea. WYT reports consulting or advisory roles with Eisai Korea, Roche Korea, and Sysmex Korea; and speaker's bureau from Bayer Korea, Gilead Korea, Samil Pharm, and Yuhan. TU reports grants or contracts from Roche and travel expenses from Gilead. EC, NM, JHS, YW, SPH, and QL report employment at Genentech and stock in Roche. CW reports employment at, and stock in, Roche. PKHC reports being Chief Medical Officer for AVATAMED; stock in AVATAMED; honoraria from AstraZeneca, Bayer, Perspectum, Roche, Sirtex, and Worrell; consulting fees from Asia-Pacific Association for the Study of the Liver, Asia-Pacific Primary Liver Cancer Expert Meeting, Bayer Liver Forum, Eastern and Western Association for Liver Tumors, Hong Kong Liver Cancer and Gastrointestinal Cancer Foundation, IQVIA, JSH International Liver Conference, Korean Radioembolization Association, Korean Radioembolization Association Webinar, Liver Cancer Collaborative Annual Scientific and Clinical Meeting, Malaysian Hepato-pancreato-biliary Congress, Malaysian Society of Interventional Radiology, Perspectum, Philippine Society of Nuclear Medicine, Roche, Singapore Hepatology Conference, State Key Laboratory of Liver Research, Taiwan Society of Interventional Radiology, and Tsinghua Medical Forum; consulting or advisory roles with AUM Biosciences, BeiGene, Omega Therapeutics, Roche, and Sirtex; grants or contracts from A*Star, AMiLi, MiRXES, National Medical Research Council, Perspectum, Roche, SingHealth Duke-NUS Programme Grant Award, SingHealth Duke-NUS Global Health Institute Pilot Research Grant, Stratificare and Sirtex; speaker's bureau from AstraZeneca, Bayer, Omega Therapeutics, Roche, and Worrell; support for attending meetings or travel expenses from Roche Diagnostics Asia Pacific and Roche Singapore; patent publication number 10202007868Q; participation in a Data Safety Monitoring Board or Advisory Board for AUM Biosciences, Genentech, IMCB, Perspectum, and Singapore-Samsung Medical Centre (SG-SMC) Joint Lab; and receipt of equipment, materials, drugs, assistance with medical writing, gifts, or other services from Roche and Sirtex. All other authors declare no competing interests., (Copyright © 2023 Elsevier Ltd. All rights reserved.)