Aim Predicting the survival of hepatocellular carcinoma (HCC) patients receiving atezolizumab and bevacizumab (Atez/bev) remains a challenge. This study aims to validate the modified albumin–bilirubin grade and α-fetoprotein score (mALF score). Methods This retrospective, multicenter study included 426 HCC patients receiving Atez/Bev. Each patient was randomized 3:2 to a training set ( n  = 255) and a validation set ( n  = 171). We investigated prognostic factors in the training set and developed an easily applicable mALF score, which was evaluated in the validation set. Results We built the mALF score using baseline mALBI grade 2b or 3 (HR 2.36, 95% CI 1.37–4.05, p  = 0.002) and α-fetoprotein ≥ 100 ng/ml (HR 2.61, 95% CI 1.49–4.55, p  < 0.001), which were identified as unfavorable prognostic factors in a multivariate analysis. The 1-year OS rates were 82.7% (95% CI 68.9–90.8) in patients who meet neither of the criteria (mALF 0 points, n  = 101), 61.7% (95% CI 44.5–74.9) in patients who meet either of the two criteria (mALF 1 point, n  = 109), and 24.6% (95% CI 9.0–44.3) in patients who meet both criteria (mALF 2 points, n = 45); the difference was statistically significant ( p  < 0.001). The median PFS in patients with mALF 0, 1, and 2 points was 9.5 months (95% CI 4.3-NA), 6.6 months (95% CI 6.0–8.0), and 3.8 months (95% CI 3.0–5.2), respectively, which amounted to a significant difference ( p  < 0.001). These results were confirmed in the validation set (1-year OS rates, 0/1/2 points = 94.2%/62.1%/46.3%, p  < 0.001; median PFS, 0/1/2 points = 9.3/6.7/4.7 months, p  = 0.018). Conclusion The mALF score can reliably predict the prognosis of HCC patients receiving Atez/Bev.