Background: There is controversy around the applicability of TMB across cancer types and reliability between assays from different manufacturers. The KEYNOTE 158 trial supported FDA approval of the Foundation Medicine test (FoundationOne CDx) at TMB ≥ 10 mut/Mb as a solid tumor companion diagnostic (CDx) for single-agent pembrolizumab in 2^nd+ line. Using a large real-world dataset with validated rwOS endpoint data, we evaluated the clinical validity of the TMB measurement by the FDA approved test in over 8,000 patients across 24 cancer types who received single agent ICI. Methods: Following a prespecified analysis plan, this study used the nationwide (US-based) de-identified Flatiron Health-Foundation Medicine clinico-genomic database (FH-FMI CGDB), with data originating from approximately 280 US cancer clinics (~800 sites of care). The overall cohort included patients from 24 cancer types of interest with adv/metastatic disease treated with single-agent anti-PD(L)1 therapy in the FH network between 1/1/2011 - 9/30/2022. This study used the TMB algorithm from the FDA-approved test supporting solid tumor CDx and composite mortality variable validated against the national death index. Relative hazards of death by TMB interval were assessed using Cox PH models adjusted for ECOG PS, prior treatment, sex, age, opioid rx pre-therapy, genetic ancestry, and socioeconomic assessment. Multi-solid tumor cohort Cox models were additionally adjusted for MSI status, with baseline hazards stratified by cancer type. Results: 8,440 patients from 24 cancer types met inclusion criteria for the overall cohort. Adjusting for aforementioned factors, the hazards of death {HR, (95% CI)} across the entire cohort, relative to TMB < 5, was 0.95 (0.89 – 1.02) for TMB 5-10, 0.79 (0.73 – 0.85) for TMB 10-20, and 0.52 (0.47 – 0.58) for TMB 20+. Adjusted Cox models comparing TMB ≥ 10 vs. TMB < 10 were pre-specified for cancer types with at least 15 death events in each group: breast 0.51 (0.35 - 0.76), colorectal 0.37 (0.27 - 0.49), cancers of unknown primary 0.52 (0.30 - 0.90), endometrial 0.38 (0.26 - 0.55), gastric 0.46 (0.34 - 0.62), head & neck 0.49 (0.33 - 0.73), melanoma 0.55 (0.43 - 0.71), non-small cell lung 0.76 (0.71 - 0.83), small cell lung 0.69 (0.44 - 1.09), and urothelial 0.63 (0.53 – 0.75). Additional individual tumor types, stratification by MSI status, and time to next treatment associations to be presented at conference. Conclusions: Across a heterogeneous cohort, and within individual cancer types with sufficient power, elevated TMB using the TMB measurement based on the FDA-approved test associated with more favorable rwOS on ICI than for similar patients with lower TMB levels. Completed and ongoing randomized controlled trials are encouraged to report out subgroup analyses comparing ICI vs. comparator arm by TMB level.