Background: The impact of a high body mass index (BMI) on hepatocellular carcinoma (HCC) has been unclear in patients with viral hepatitis and liver cirrhosis (LC). Our aim is to investigate the association between BMI and HCC in patients with viral hepatitis and LC. Methods: We conducted a population-based cohort study. In total, 156,143 Korean adults who underwent routine health examinations during 2003-2006 were followed up until December 2019 via linkage to the national hospital discharge records. Multivariable-adjusted hazard ratios (HRs) per each 5 kg/m² increase in BMI for HCC risk were calculated using Cox proportional hazard models. Results: BMI was positively associated with HCC in hepatitis B virus (HBV) patients with BMI range of 27.5~29.9 kg/m² and BMI ≥30 kg/m² and LC patients with a BMI range of 27.5~29.9 kg/m². Assuming linear associations, HRs were 1.17 for HBV, 1.08 for hepatitis C virus (HCV), and 1.11 for LC. In the subgroup analysis for the LC, the HR were 1.19 in the HBV-LC, 1.38 in the HCV-LC, 1.36 in the alcoholic LC, and 1.59 in the fatty liver LC. Women showed stronger positive associations between BMI and HCC than men in the HBV (HR, 1.45 vs 1.06) and LC (1.27 vs. 1.02). The positive association seemed to be stronger in younger age ( < 65 years) in HBV (1.17 vs 1.05) and HCV (1.33 vs 0.77) patients compared to older age ( > 65 years). Conclusions: In the BMI range of ≥25 kg/m², a higher BMI was associated with an increased risk of HCC in LC and HBV. The associations between BMI and HCC risk are different according to liver disease, sex and age.