Background & Aims Carbon‐ion radiation therapy has shown encouraging results in hepatocellular carcinoma patients in single‐centre studies. We evaluated the effectiveness and safety of short‐course carbon‐ion radiation therapy for hepatocellular carcinoma in a multicentre study conducted by the Japan Carbon Ion Radiation Oncology Study Group. Methods Consecutive hepatocellular carcinoma patients who were treated with carbon‐ion radiation therapy in four or fewer fractions at four Japanese institutions between April 2005 and November 2014 were analysed retrospectively. The primary outcome was overall survival; secondary outcomes were local control rate, treatment‐related toxicity and radiation‐induced liver disease. Results A total of 174 patients were included in this study. Prescribed carbon‐ion radiation therapy doses were (relative biological effectiveness): 48.0 Gy in two fractions (n = 46), and 52.8 Gy (n = 108) and 60.0 Gy (n = 20) in four fractions. The median follow‐up period was 20.3 (range, 2.9‐103.5) months. The overall survival and local control rates at 1, 2 and 3 years were 95.4%, 82.5% and 73.3%; and 94.6%, 87.7% and 81.0% respectively. Multivariate analysis revealed that Eastern Cooperative Oncology Group performance status 1‐2, Child‐Pugh class B, maximum tumour diameter ≥3 cm, multiple tumours and serum alpha foetoprotein level >50 ng/mL were significant prognostic factors of overall survival. No treatment‐related death occurred during the follow‐up period. Grades 3 or 4 treatment‐related toxicities were observed in 10 patients (5.7%); radiation‐induced liver disease was observed in three patients (1.7%). Conclusions Short‐course carbon‐ion radiation therapy is a safe, effective and potentially curative therapy for hepatocellular carcinoma. See Editorial on Page 2132