Background The aim of this study was to investigate the influence of serum iron levels in advanced gastric cancer (GC) patients treated with programmed cell death protein-1 (PD-1) inhibitors. Methods We retrospectively reviewed 149 GC patients who were treated with PD-1 inhibitors at our center. Clinicopathological characteristics, laboratory data, and clinical outcomes were analyzed. Results Multivariate analysis showed that Eastern Cooperative Oncology Group performance status (ECOG PS), histological subtype, and baseline serum iron levels were independent prognostic factors for overall survival (OS), while ECOG PS, multiple metastatic sites, and baseline serum iron levels were independent prognostic factors for progression-free survival (PFS). Patients with baseline low serum iron levels (LSI) had a significantly shorter median OS and PFS compared to patients with normal serum iron levels (NSI) (Median OS: 7 vs. 14 months, p = 0.001; median PFS: 3 vs. 5 months, p = 0.005). Patients with baseline LSI had a disease control rate (DCR) of 58.3% at 2 months after PD-1 inhibitor initiation (M2), compared to 81.1% in patients with NSI (p = 0.005). Patients with baseline LSI had a DCR of 43.8% at 4 months, compared to 64.2% in patients with NSI (p = 0.017). Conclusions LSI was associated with worse OS, PFS, and DCR in GC patients treated with PD-1 inhibitors and might be a quick and efficient biomarker to predict the efficacy of PD-1 inhibitors. Keywords: Gastric cancer, Serum iron, Immune checkpoint inhibitor, Biomarker