It was recently reported that hepatocellular carcinoma (HCC) patients with non-alcoholic steatohepatitis (NASH) are not responsive to immune-checkpoint inhibitor (ICI) treatment. The present study aimed to evaluate the therapeutic efficacy of lenvatinib in patients with non-alcoholic fatty liver disease (NAFLD)/NASH-related unresectable-HCC (u-HCC). Five hundred thirty u-HCC patients with Child–Pugh A were enrolled, and divided into the NAFLD/NASH (n = 103) and Viral/Alcohol (n = 427) groups. Clinical features were compared in a retrospective manner. Progression-free survival (PFS) was better in the NAFLD/NASH than the Viral/Alcohol group (median 9.3 vs. 7.5 months, P = 0.012), while there was no significant difference in overall survival (OS) (20.5 vs. 16.9 months, P = 0.057). In Cox-hazard analysis of prognostic factors for PFS, elevated ALT (≥ 30 U/L) (HR 1.247, P = 0.029), modified ALBI grade 2b (HR 1.236, P = 0.047), elevated AFP (≥ 400 ng/mL) (HR 1.294, P = 0.014), and NAFLD/NASH etiology (HR 0.763, P = 0.036) were significant prognostic factors. NAFLD/NASH etiology was not a significant prognostic factor in Cox-hazard analysis for OS (HR0.758, P = 0.092), whereas AFP (≥ 400 ng/mL) (HR 1.402, P = 0.009), BCLC C stage (HR 1.297, P = 0.035), later line use (HR 0.737, P = 0.014), and modified ALBI grade 2b (HR 1.875, P < 0.001) were significant. Lenvatinib can improve the prognosis of patients affected by u-HCC irrespective of HCC etiology or its line of treatment.