Background: With the advent of immunotherapy as an option for gastric, gastroesophageal junction (GEJ), or esophageal cancer, guidelines recommend testing for mismatch repair (MMR) or microsatellite instability (MSI) and programmed death-ligand 1 (PD-L1) at time of advanced cancer diagnosis. However, little real-world information is available on testing patterns and results for MMR/MSI and PD-L1. Methods: This study used the nationwide Flatiron Health electronic health record-derived de-identified database. Starting with a subset of 1594 patients (pts) from the Advanced Gastric/ Esophageal Cancer database who had initiated first-line (1L) treatment 01Jan2017-31Aug2020 and for whom additional abstraction for details on MMR/MSI and PD-L1 testing was conducted, adult pts with ≥3 months (mos) of follow-up were selected. 1L initiation was index date. For each biomarker, pts were classified as tested vs not. Pt and disease characteristics and test results were summarized. Testing relative to line of therapy and PD-1 inhibitor use were evaluated (data will be presented later). Results: Of 1142 eligible pts, 88% were initially diagnosed with advanced disease and 92% treated in community practice. 582 (51%) were tested for MMR/MSI and 571 (50%) for PD-L1; 451 pts were tested for both. MMR/MSI testing rates were 59% for gastric, 51% for GEJ, and 46% for esophageal. PD-L1 testing rates were similar across primary tumor sites (48-54%), but lower in those with squamous histology (38% for esophageal/GEJ). For both, testing increased over time with ̃40% in 2017 and 60% in 2020. For both, median number of tests per pt was 1 (range 1-6 for MMR/MSI, 1-4 for PD-L1). For both, mean time from initial diagnosis to first test was approximately 4 mos (> 5 mos in pts in 2017 and < 2 mos for those in 2020). Majority (> 68%) of all tests used tissue from primary tumor site. 46% of all MMR/MSI tests were performed with immunohistochemistry and 43% with next-generation sequencing; 5% were dMMR/MSI-H. 71% of all PD-L1 tests were performed using the commercial 22C3 assay. Reported composite positive score (CPS) results are summarized below. Conclusions: Rates of MMR/MSI and PD-L1 biomarker testing are increasing and time relative to initial diagnosis is shortening. A high proportion of pts tested positive for PD-L1 with CPS > 1. PD-L1 CPS varies by primary tumor site, histology, and tissue collection site. In the new era of 1L immunotherapy, increased rates of testing and reduced time to testing are likely to make more meaningful impact on treatment decisions.