Background: Immune checkpoint inhibitors (ICIs) have emerged as the first-line therapy for unresectable hepatocellular carcinoma (HCC). However, their administration poses the risk of immune-related adverse events (irAEs). Recent data suggests that the occurrence of irAEs may be associated with a favorable prognosis. Nevertheless, limited studies have explored the relationship between irAE severity or specific symptoms and clinical outcomes. Therefore, the objective of this study was to investigate and analyze the association between these adverse events and the clinical benefits in patients with unresectable HCC treated with immune checkpoint inhibitors. Methods: We enrolled 130 patients with unresectable HCC treated with atezolizumab plus bevacizumab between November 2022 and January 2023 at a single center. We investigated the relationship between irAEs and both response rate and post-treatment outcomes. Results: Out of the 130 patients, 36 (27.7%) developed irAEs. Patients with irAEs exhibited a significantly longer progression-free survival (PFS) compared to the non-irAE group, with a median PFS of 8.9 months compared to 4.6 months (p < 0.01). There was no difference in overall survival with or without irAEs. The irAE group demonstrated significantly higher disease control rate (DCR) compared to the non-irAE group (97.0% vs 65.5%, p < 0.01). The analysis by the severity of irAE revealed that the grade 1/2 group exhibited significantly longer PFS (7.9 months vs 4.6 months, p =0.007) and higher DCR (100% vs 65.5%, p < 0.01) compared to the non-irAE group. Furthermore, hypothyroidism was correlated with a favorable PFS (8.9 months vs 5.4 months, p = 0.02), DCR (100% vs 71.3%, p = 0.03) and overall response rate (58.3% vs 18.5%, p = 0.005). Conclusions: The presence of irAEs is associated with prolonged PFS and higher DCR. Specifically, mild irAEs (Grade 1/2) and hypothyroidism displayed prolonged PFS and higher DCR.