The sphingosine 1-phosphate receptor 1 (S1P₁) is abundant in endothelial cells, where it regulates vascular development and microvascular barrier function. In investigating the role of endothelial cell S1P₁ in adult mice, we found that the endothelial S1P₁ signal was enhanced in regions of the arterial vasculature experiencing inflammation. The abundance of proinflammatory adhesion proteins, such as ICAM-1, was enhanced in mice with endothelial cell–specific deletion of S1pr1 and suppressed in mice with endothelial cell–specific overexpression of S1pr1, suggesting a protective function of S1P₁ in vascular disease. The chaperones ApoM⁺HDL (HDL) or albumin bind to sphingosine 1-phosphate (S1P) in the circulation; therefore, we tested the effects of S1P bound to each chaperone on S1P₁ signaling in cultured human umbilical vein endothelial cells (HUVECs). Exposure of HUVECs to ApoM⁺HDL-S1P, but not to albumin-S1P, promoted the formation of a cell surface S1P₁–β-arrestin 2 complex and attenuated the ability of the proinflammatory cytokine TNFα to activate NF-κB and increase ICAM-1 abundance. Although S1P bound to either chaperone induced MAPK activation, albumin-S1P triggered greater G_i activation and receptor endocytosis. Endothelial cell–specific deletion of S1pr1 in the hypercholesterolemic Apoe^−/− mouse model of atherosclerosis enhanced atherosclerotic lesion formation in the descending aorta. We propose that the ability of ApoM⁺HDL to act as a biased agonist on S1P₁ inhibits vascular inflammation, which may partially explain the cardiovascular protective functions of HDL.