Significance Soluble klotho is the shed ectodomain of the antiaging membrane protein α-klotho that exhibits pleiotropic actions, including down-regulation of growth factor-driven PI3K signaling, contributing to lifespan prolongation, cardioprotection, and tumor inhibition. Whether membrane receptors exist for soluble klotho is unknown. We identify lipid rafts as receptors for soluble klotho. We show klotho binds specific sialic acid residues of gangliosides highly enriched in the outer leaflet of lipid rafts. Klotho binding to gangliosides modulates lipid organization and inhibits lipid raft-dependent PI3K signaling. In vivo, klotho-deficient mouse hearts have heightened raft-dependent PI3K signaling vs. wild-type hearts. We reveal a novel physiological regulator of lipid raft structure and function, and open new research to understand pleiotropic effects of klotho in cell signaling and metabolism.