Objectives Aortic valve (AV) calcification occurs via a pathophysiological process that includes lipoprotein deposition, inflammation, and osteoblastic differentiation of valvular interstitial cells. Here, we investigated the association between endoplasmic reticulum (ER) stress and AV calcification. Approach and Results We identified ER stress activation in AV of patients with calcified AV stenosis. We generated an AV calcification model in hypercholesterolemic rabbits and mice, respectively, and found marked AV ER stress induction. Classical ER stress inhibitor, tauroursodeoxycholic acid, administration markedly prevented AV calcification, and attenuated AV osteoblastic differentiation and inflammation in both rabbit and mouse models of AV calcification via inhibition of ER stress. In cultured valvular interstitial cells (VICs), we found that oxidized low density lipoprotein (oxLDL) caused ER stress in a cytosolic [Ca](2+)(i)-dependent manner. OxLDL promoted osteoblastic differentiation via ER stress-mediated protein kinase-like ER kinase/activating transcription factor 4/osteocalcin and inositol-requiring transmembrane kinase and endonuclease-1 (IRE1)/spliced X-box-binding protein 1/Runx2 pathway, and induced inflammatory responses through IRE1/c-Jun N-terminal kinase and IRE1/nuclear factor kappa-light-chain-enhancer of activated B cells signaling in VICs. Inhibition of ER stress by either tauroursodeoxycholic acid or 4-phenyl butyric acid could both suppress oxLDL-induced osteoblastic differentiation and inflammatory responses in VICs. Conclusions These data provide novel evidence that ER stress participates in AV calcification development, and suggest that ER stress may be a novel target for AV calcification prevention and treatment.