Background <p>Bemarituzumab, a monoclonal antibody targeting fibroblast growth factor receptor 2b (FGFR2b), is under evaluation in phase 3 trials of gastric cancer (GC) trials. However, data on the characteristics, prognostic significance, and heterogenous expression patterns of FGFR2b overexpression in GC remain limited. Therefore, this study aims to investigate the clinicopathologic characteristics and survival outcomes of FGFR2b-positive GC, along with the expression concordance across biopsy, surgical, and metastatic specimens.</p> Methods <p>This retrospective study included 466 patients with stages I–IV GC. Biopsy-surgical and primary-metastatic specimen pairs were available for 163 and 135 patients, respectively. FGFR2b overexpression was defined as moderate-to-strong membranous and/or cytoplasmic expression in ≥ 10% of tumor cells. <i>FGFR2</i> amplification was evaluated using chromogenic in situ hybridization.</p> Results <p>FGFR2b overexpression was observed in 4.1% of patients, with 14/341 surgical specimens (4.1%), 3/284 gastric biopsies (1.1%), and 4/135 metastatic specimens (3.0%). FGFR2b overexpression correlated with deeper invasion and perineural invasion in resectable GC. However, it did not influence survival outcomes in resectable or metastatic GC. Among 163 biopsy-surgical pairs, FGFR2b overexpression was observed in only one pair (0.6%). Similarly, among 135 paired primary-metastatic specimens, FGFR2b overexpression was observed in one (0.7%). <i>FGFR2</i> gene amplification occurred in 16/17 (94.1%) cases with FGFR2b overexpression.</p> Conclusion <p>Significant intratumoral and intrapatient heterogeneity is observed in FGFR2b overexpression. Given this variability in expression levels, a single endoscopic biopsy may not accurately assess FGFR2b overexpression.</p> Mini-abstract <p>The FGFR2b positivity rate in gastric cancers was 4.1%, likely due to the substantial heterogeneity in its expression.</p>