Radiotherapy is widely used in the treatment of lung cancer; a number of intrinsic and extrinsic mechanisms of resistance exist, including hypoxia. Targeting metabolic pathways that support redox homeostasis has been proposed as a strategy to enhance radiosensitivity. The serine synthesis pathway enzyme, phosphoglycerate dehydrogenase (PHGDH), has been implicated in resistance to several anticancer therapies; however, its role in radiotherapy response is poorly defined. We show that PHGDH expression positively correlates with established hypoxia gene expression signatures in lung adenocarcinoma and squamous cell carcinoma patient datasets and is induced under hypoxia (<0.1% O2) in lung cancer cell lines. Hypoxic induction of PHGDH appears mediated by both HIF-1 and PERK signalling, linking PHGDH regulation to hypoxia and the unfolded protein response. Using CRISPR-Cas9 PHGDH knockout models, we demonstrate that loss of PHGDH does not enhance radiosensitivity in normoxia or hypoxia when extracellular serine and glycine are available at physiological or supraphysiological concentrations. Radiosensitisation is observed only under complete serine/glycine deprivation. Unexpectedly, PHGDH loss confers increased tolerance to hypoxic stress, associated with elevated glycolytic flux, increased lactate production, accelerated HIF-1α stabilisation and enhanced expression of hypoxia-inducible genes. Overall, these data indicate that targeting PHGDH alone is unlikely to impact radioresistance in lung cancer under physiologically relevant nutrient conditions and may instead promote metabolic adaptation to hypoxia. This highlights the importance of microenvironmental context when evaluating metabolic targets for combination with radiotherapy.