Accumulation of unfolded proteins in the endoplasmic reticulum triggers the unfolded protein response (UPR), an adaptive signal transduction pathway aimed at reinstating cellular homeostasis, or, if that fails, at triggering of apoptosis. The UPR plays a key role in a variety of disorders (including diabetes, neurodegenerative disorders, and inflammatory processes) and has been implicated in cancer progression and resistance to chemotherapy. However, the mechanisms and pathways by which the UPR contributes to chemoresistance are only poorly understood. We have employed a multi-omics approach to monitor changes to gene expression after induction of the UPR with two different compounds, probing in parallel the transcriptome, the proteome, and changes to translation. Stringent filtering reveals the induction of 267 genes (the UPR regulon), many of which have not previously been implicated in stress response pathways. We experimentally demonstrate that UPR-mediated translational control via phosphorylation of eukaryotic translation initiation factor 2 alpha (eIF2a) and subsequent induction of activating transcription factor 4 (ATF4) causes an up-regulation of enzymes involved in a pathway that diverts intermediate metabolites from glycolysis to fuel mitochondrial one-carbon metabolism. This metabolic rewiring of the cells results in resistance to treatment with the widely-used folate anti-metabolites Methotrexate and Permetrexed.