Atezolizumab (anti-programmed death-ligand 1; anti-PD-L1) in conjunction with bevacizumab (anti-vascular endothelial growth factor; anti-VEGF) has become the established standard first-line systemic treatment for unresectable hepatocellular carcinoma (HCC). Despite an improved objective response rate (ORR) of 27%, the majority of patients face HCC progression and liver failure \[Finn et al., N Engl J Med 2020\]. Developing a new combined treatment strategy to overcome resistance to anti-PD-L1 and anti-VEGF is essential to improve patient outcomes. Radiation treatment (RT) is notably effective in managing localized solid tumors and is a fundamental component of unresectable HCC treatment. Recent retrospective cohorts have demonstrated that proton RT targeting all hepatic tumors, along with PD-L1/programmed death-1 (PD-1) blockade, enhances ORR and progression-free survival for unresectable HCC patients, displaying a favorable safety profile (Su et al., Am J Cancer Res. 2022). Our preclinical study (Hsieh et al., Sci Immunol 2022) showcased that RT combined with PD-L1/PD-1 blockade stimulates immunogenic cell death and antigen cross-presentation in murine tumor models, promoting systemic antitumor T cell responses. Nonetheless, it is crucial to verify whether the combined therapy of proton RT, atezolizumab, and bevacizumab triggers synergistic antitumor effects and systemic immune activation in clinical trials for unresectable HCC. This phase II non-randomized trial aims to prospectively evaluate therapeutic efficacy, safety, and immunological responses in patients with unresectable HCC treated with atezolizumab/bevacizumab combined with proton radiotherapy.