Background: There is an unmet need for additional therapies to treat patients (pts) with locally advanced (LA) unresectable or metastatic gastric or gastroesophageal junction (mG/GEJ) adenocarcinoma. Claudin-18.2 (CLDN18.2), a new validated target in these pts, is expressed in normal gastric mucosa cells and often retained in G/GEJ tumor cells. There are limited data on the global prevalence of CLDN18.2 in tumors of pts with LA unresectable or mG/GEJ adenocarcinoma. The SPOTLIGHT (NCT03504397) and GLOW (NCT03653507) phase 3 studies demonstrated clinically meaningful and statistically significant improvement in PFS and OS with the CLDN18.2-targeted antibody zolbetuximab + chemotherapy (mFOLFOX6 or CAPOX, respectively) vs placebo + chemotherapy as 1L therapy in pts with CLDN18.2+/HER2− disease. We report the biomarker analysis of these studies. Methods: Pts with LA unresectable or mG/GEJ adenocarcinoma were screened for CLDN18.2 status by IHC before enrollment. Tumors were defined as CLDN18.2+ if they had ≥75% of tumor cells with moderate-to-strong membranous CLDN18 staining per the investigational VENTANA CLDN18 (43-14A) RxDx Assay. HER2 status was evaluated per central or local assessment. Ad hoc PD-L1 IHC was performed via the Dako PD-L1 IHC 28-8 pharmDx assay for a subgroup of enrolled pts. Results exclude pts from mainland China and pts missing data for disease type, Lauren classification, or tumor collection site. Results: Across SPOTLIGHT and GLOW, 3576 pts had valid CLDN18 IHC results; 1399 (39.1%) had CLDN18.2+ tumors. CLDN18.2 prevalence was 43.7% (513/1175) in female pts and 36.9% (886/2401) in male pts. CLDN18.2 prevalence was 44.0% (671/1524) in pts in Europe/Middle East, 37.7% (183/485) in pts in N. America, and 36.5% (479/1314) in pts in Asia Pacific. CLDN18.2 prevalence was 41.0% (1056/2576) in pts with gastric adenocarcinoma and 37.3% (302/809) in pts with GEJ adenocarcinoma. CLDN18.2 prevalence trended higher in pts with diffuse (48.9%, 479/980) vs intestinal (38.9%, 265/682) disease. CLDN18.2 prevalence was 41.1% (175/426) in tumors collected from metastatic sites and 38.6% (1195/3094) in tumors collected from primary sites. CLDN18.2 prevalence was similar across other clinical and histopathological characteristics. Of 2908 HER2− pts with valid CLDN18 IHC results, 1264 (43.5%) had CLDN18.2+ tumors. Of 599 enrolled pts tested for PD-L1 expression, 104 (17.4%) had a PD-L1 CPS ≥5. Conclusions: The SPOTLIGHT and GLOW phase 3 studies are the largest data sources to date for determining global CLDN18.2 prevalence. CLDN18.2 is a high prevalence biomarker, with high tumor expression in ~35–40% of pts with LA unresectable or mG/GEJ adenocarcinoma, for whom zolbetuximab + chemotherapy represents a potential 1L therapy. Clinical trial information: NCT03504397, NCT03653507.