There is convincing evidence that endoplasmic reticulum (ER) stress is implicated in the pathogenesis of diabetes and its complications; however, the mechanisms are not fully understood. This study aimed to dissect the role and signalling pathways of activating transcription factor 4 (ATF4) in ER-stress-associated endothelial inflammation and diabetic retinopathy.ER stress and ATF4 activity were manipulated by complementary pharmacological and genetic approaches in cultured retinal endothelial (TR-iBRB) cells. Diabetes was induced by streptozotocin in heterozygous Atf4 knockout and wild-type mice. ER stress markers, inflammatory cytokines and adhesion molecules, activation of the signal transducer and activator of transcription 3 (STAT3) pathway, and retinal vascular permeability were measured.High-glucose treatment resulted in rapid induction of ER stress, activation of ATF4, and increased production of inflammatory factors in TR-iBRB cells. Suppressing ER stress or inhibiting ATF4 activity markedly attenuated high-glucose-induced production of intercellular adhesion molecule 1, TNF-α and vascular endothelial growth factor. Conversely, enhancing ER stress or overexpressing Atf4 was sufficient to induce endothelial inflammation, which was, at least in part, through activation of the STAT3 pathway. Furthermore, knockdown of the Stat3 gene or inhibiting STAT3 activity restored ER homeostasis in cells exposed to high glucose and prevented ATF4 activation, suggesting that STAT3 is required for high-glucose-induced ER stress. Finally, we showed that downregulation of Atf4 significantly ameliorated retinal inflammation, STAT3 activation and vascular leakage in a mouse model of type 1 diabetes.Taken together, our data reveal a pivotal role of ER stress and the ATF4/STAT3 pathway in retinal endothelial inflammation in diabetic retinopathy.