Activating transcription factors (ATFs), members of the adaptive-response gene family, participate in cellular processes to aid adaptations in response to extra and/or intracellular changes. In this study, we observed that one of the ATFs, Activating transcription factor 3 (ATF3), is upregulated under hypoxia via alterations in the epigenetic landscape of its promoter, followed by transcriptional upregulation. Under hypoxic conditions, Hypoxia-inducible factor 1-alpha (HIF1ɑ) alleviates methylation at the ATF3 promoter by recruiting TET1 and induces ATF3 transcription. In addition, our RNA-seq analysis showed that ATF3 globally affects transcription under hypoxia and controls the processes of EMT and cancer invasion by stimulating the transcription of Prolyl 4-Hydroxylase Subunit Alpha 1 (P4HA1), an enzyme which enhances invasion-conducive extracellular matrix (ECM) under hypoxic conditions. Prolyl hydroxylases play a critical role in the hydroxylation and deposition of collagen in the extracellular matrix (ECM) during the evolution of cancer, which is necessary for metastasis. Importantly, P4HA1 undergoes alternative splicing under hypoxia, where the inclusion of exon 9a is increased. Interestingly, involvement of ATF3 in P4HA1 splicing was also evident, as binding of ATF3 at intron 9a led to demethylation of this DNA region via recruitment of TET1. Furthermore, we also show that the demethylated DNA region of intron 9a then becomes accessible to CCCTC-binding factor (CTCF). Thus, a cascade of demethylation via ATF3 recruited TET1, followed by increased RNA Pol II pause at intron 9a via CTCF, leads to inclusion of exon 9a. The P4HA1 9a isoform leads to enhanced invasion under hypoxic conditions by increasing deposition of collagen in the ECM. These results reveal a novel hypoxia-induced HIF1ɑ-ATF3-P4HA1 axis which can potentially be exploited as a therapeutic target to impede EMT and ultimately breast cancer invasion. Hypoxia induced ATF3 regulates P4HA1 expression and alternative splicing to promote breast cancer invasion.