DNA damaging chemotherapy and radiation are widely used standard-of-care modalities for the treatment of cancer. Nevertheless, the outcome for many patients remains poor and this may be attributed, at least in part, to highly effective DNA repair mechanisms. Ataxia-telangiectasia mutated and Rad3-related (ATR) is a key regulator of the DNA-damage response (DDR) that orchestrates the repair of damaged replication forks. ATR is a serine/threonine protein kinase and ATR kinase inhibitors potentiate chemotherapy and radiation. The ATR kinase inhibitor VX-970 (NSC 780162) is in clinical development in combination with primary cytotoxic agents and as a monotherapy for tumors harboring specific mutations. We have developed and validated an LC–MS/MS assay for the sensitive, accurate and precise quantitation of VX-970 in human plasma. A dilute-and-shoot method was used to precipitate proteins followed by chromatographic separation with a Phenomenex Polar-RP 80 Å (4 μm, 50 × 2 mm) column and a gradient acetonitrile-water mobile phase containing 0.1% formic acid from a 50 μL sample volume. Detection was achieved using an API 4000 mass spectrometer using electrospray positive ionization mode. The assay was linear from 3 to 5,000 ng/mL, proved to be accurate (94.6–104.2%) and precise (<8.4% CV), and fulfilled criteria from the FDA guidance for bioanalytical method validation. This LC–MS/MS assay will be a crucial tool in defining the clinical pharmacokinetics and pharmacology of VX-970 as it progresses through clinical development.