Sphingolipids (SL) are both membrane building blocks and potent signaling molecules regulating a variety of cellular functions, in both physiological and pathological conditions. Sphingosine-1-phosphate (S1P) is a secreted bioactive sphingolipid acting through G-protein coupled receptors namely S1PR1 to S1PR5, with major impact on cardiovascular and immune systems. Current drugs targeting S1PRs have been approved by the FDA to treat autoimmune conditions. However, these drugs manifest cardiovascular-adverse effects, suggesting that knowledge gaps on S1P functions remain, limiting our ability to exploit this pathway therapeutically. Disruption of sphingolipid homeostasis and signaling has been implicated in cancer, diabetes, cardiovascular and autoimmune diseases. Yet, under normal physiology, sphingolipid levels are tightly-regulated. Mechanisms governing cellular sensing of SL, and according regulation of sphingolipid biosynthesis, remain largely unknown. Serine palmitoyltransferase (SPT), which catalyzes the first and rate limiting step of sphingolipid de novo biosynthesis, is functionally antagonized by Orosomucoid like 1, 2 and 3 proteins (ORMDLs). Here, we identify S1P as the key sphingolipid sensed by cells via S1PRs, which stabilizes ORMDLs to negatively regulate SPT and maintain SL homeostasis. These findings reveal the S1PR/ORMDLs axis as the sensor-effector unit regulating SPT activity accordingly. Mechanistically, the hydroxylation of ORMDLs at Pro137 allows a constitutive degradation of ORMDLs via ubiquitin-proteasome pathway, therefore preserving SPT activity at steady state. The increase of S1P-S1PR signaling stabilizes ORMDLs, which downregulates SPT activity to maintain SL within a physiological range. The disruption of the S1PR/ORMDL axis results in ceramide accrual and endothelial dysfunction, which is an early event in the onset of cardio- and cerebrovascular diseases. It is likely that the disruption of S1P-ORMDL-SPT signaling might be implicated also in the pathogenesis of other conditions such as diabetes, aging, cancer, and cardiomyopathy, and neurodegeneration characterized by deranged SL metabolism.