Endoplasmic reticulum (ER) stress and the subsequent unfolded protein response (UPR) have been implicated in β-cell apoptosis in type 1 and type 2 diabetes. However, the UPR is also a fundamental mechanism required for β-cell adaptation and survival. The mechanisms regulating the transition from adaptive to apoptotic UPR remain to be clarified. Here, we investigated the relationships between XBP1, CHOP and JNK in the transition from adaptive to apoptotic UPR and β-cell death in models of type 1 and type 2 diabetes. XBP1 inhibition potentiated apoptosis induced by pro-inflammatory cytokines or the saturated fatty acid palmitate in MIN6 β-cells. This response was prevented by CHOP inhibition. XBP1 expression increased with time in islets from diabetes-resistant ob/ob mice, but declined in diabetes-prone db/db mice. IRE1/XBP1 inhibition led to alterations in islets from ob/ob mice that resemble those found in diabetes, including increases in inflammation and antioxidant gene expression and apoptosis. Similarly, IRE1/XBP1 inhibition increased apoptosis in islets from NOD mice. On the other hand, JNK inhibition: 1) increased adaptive UPR and reduced apoptosis in islets from diabetic db/db mice, and 2) restored adaptive UPR while protecting against apoptotic UPR gene expression and apoptosis following cytokine exposure. These findings suggest that the balance between XBP1-mediated adaptive and CHOP-dependent apoptotic UPR is critically important for β-cell survival during ER stress. JNK activation regulates the transition from adaptive to apoptotic UPR, thus providing a mechanism for β-cell propensity to apoptosis rather than ER stress adaptation in type 1 and type 2 diabetes.