Arsenic is a mitochondrial toxin, and its derivatives, such as arsenic trioxide (ATO), can trigger endoplasmic reticulum (ER) and the associated unfolded protein response (UPR). Here, we show that arsenic induction of the UPR triggers ATF4, which is involved in regulating this ER-mitochondrial crosstalk that is important for the molecular pathogenesis of arsenic toxicity. Employing ATF4+/+ and ATF4−/− MEFs, we show that ATO induces UPR and impairs mitochondrial integrity in ATF4+/+ MEF cells which is largely ablated upon loss of ATF4. Following ATO treatment, ATF4 activates NADPH oxidase by promoting assembly of the enzyme components Rac-1/P47phox/P67phox, which generates ROS/superoxides. Furthermore, ATF4 is required for triggering Ca++/calpain/caspase-12-mediated apoptosis following ATO treatment. The IP3R inhibitor attenuates Ca++/calpain-dependent apoptosis, as well as reduces m-ROS and MMP disruption, suggesting that ER-mitochondria crosstalk involves IP3R-regulated Ca++ signaling. Blockade of m-Ca++ entry by inhibiting m-VDAC reduces ATO-mediated UPR in ATF4+/+ cells. Additionally, ATO treatment leads to p53-regulated mitochondrial apoptosis, where p53 phosphorylation plays a key role. Together, these findings indicate that ATO-mediated apoptosis is regulated by both ER and mitochondria events that are facilitated by ATF4 and the UPR. Thus, we describe novel mechanisms by which ATO orchestrates cytotoxic responses involving interplay of ER and mitochondria.