This retrospective study examined patients with advanced or recurrent gastric cancer who received nivolumab monotherapy as a third-line or later treatment at Saitama Cancer Center (Saitama, Japan) between January 2018 and June 2022. The aim was to assess the incidence of immune-related adverse events (irAEs) and explore their association with patient prognosis. Patients who experienced Grade 1 or higher irAEs, as defined by the Common Terminology Criteria for Adverse Events version 5.0, were categorized into the irAE group, whereas those without irAEs comprised the non-irAE group. Among the 134 patients analyzed, 38 (28.4%) developed irAEs, with skin disorders and hypothyroidism being the most common. Overall survival (OS) was significantly longer in the irAE group (median OS: 13.23 months; 95% confidence interval [CI]: 6.94–24.83) than in the non-irAE group (median OS: 4.83 months; 95% CI: 3.76–5.63) (p < 0.001). Significant differences were also observed between the two groups in terms of primary tumor resection and disease stage (p < 0.05). Multivariate analysis identified the presence of irAEs (hazard ratio [HR]: 0.23) and subsequent treatments after nivolumab (HR: 0.29) as favorable prognostic factors. Conversely, elevated alkaline phosphatase levels were associated with poorer outcomes (HR: 1.64) (p < 0.001). These findings suggest that the development of irAEs during nivolumab monotherapy may be linked to improved survival outcomes in patients with advanced or recurrent gastric cancer.