Rationale The endoplasmic reticulum (ER) is a major intracellular Ca2+ store in endothelial cells (ECs). The Ca2+ concentration in the ER greatly contributes to the generation of Ca2+ signals that regulate endothelial functions. Many proteins, including stromal interaction molecule 1/2 (STIM1/2), Orai1/2/3, and sarcoplasmic/ endoplasmic reticulum Ca2+-ATPase 3 (SERCA3), are involved in the ER Ca2+ refilling after store depletion in ECs. Objective This study is designed to examine the role of Ca2+ in the ER in coronary endothelial dysfunction in diabetes. Methods and Results Mouse coronary ECs (MCECs) isolated from diabetic mice exhibited (1) a significant decrease in the Ca2+ mobilization from the ER when the cells were treated by SERCA inhibitor, and (2) significant downregulation of STIM1 and SERCA3 protein expression in comparison to the controls. Overexpression of STIM1 restored (1) the increase in cytosolic Ca2+ concentration due to Ca2+ leak from the ER in diabetic MCECs, (2) the Ca2+ concentration in the ER, and (3) endothelium-dependent relaxation that was attenuated in diabetic coronary arteries. Conclusions Impaired ER Ca2+ refilling in diabetic MCECs, due to the decrease in STIM1 protein expression, attenuates endothelium-dependent relaxation in diabetic coronary arteries, while STIM1 overexpression has a beneficial and therapeutic effect on coronary endothelial dysfunction in diabetes.