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Determination of target engagement for candidate drug molecules in the native cellular environment is a significant challenge for drug discovery programs. The cellular thermal shift assay (CETSA) has emerged as a powerful tool for determining compound target engagement through measurement of changes to a protein's thermal stability upon ligand binding. Here, we present a HiBiT thermal shift assay (BiTSA) that deploys a quantitative peptide tag for determination of compound target engagement in the native cellular environment using a high throughput, plate-based luminescence readout. We demonstrate that BiTSA can rapidly assess cellular target engagement of small molecule ligands against their cognate targets and highlight two applications of BiTSA for differentiating small molecules targeting mutant KRAS and TP53., Competing Interests: The authors declare the following competing financial interest(s): Authors are current or former employees of Merck & Co., Inc. and MSD (Merck, Sharp and Dohme Corp.)., (© 2021 American Chemical Society.)