Class Ia phosphoinositide 3-kinase (PI3K), an essential mediator of the metabolic actions of insulin, is composed of a catalytic (p110α or p110β) and regulatory (p85αα, p85βα or p55α) subunit. Here we show that p85αα interacts with X-box–binding protein-1 (XBP-1), a transcriptional mediator of the unfolded protein response (UPR), in an endoplasmic reticulum (ER) stress-dependent manner. Cell lines with knockout or knockdown of p85αα show marked alterations in the UPR, including reduced ER stress–dependent accumulation of nuclear XBP-1, decreased induction of UPR target genes and increased rates of apoptosis. This is associated with a decreased activation of inositol-requiring protein-1α (IRE1α) and activating transcription factor-6αα (ATF6α). Mice with deletion of p85α in liver (L-Pik3r1^−/−) show a similar attenuated UPR after tunicamycin administration, leading to an increased inflammatory response. Thus, p85αα forms a previously unrecognized link between the PI3K pathway, which is central to insulin action, and the regulation of the cellular response to ER stress, a state that when unresolved leads to insulin resistance.