The divalent cation Ca²⁺ is a key component in many cell signaling and membrane trafficking pathways. Ca²⁺ signal transduction commonly occurs through interaction with protein partners. However, in this study we show a novel mechanism by which Ca²⁺ may impact membrane structure. We find an asymmetric concentration of Ca²⁺ across the membrane triggers deformation of membranes containing negatively charged lipids such as phosphatidylserine (PS) and phosphatidylinositol-4,5-bisphosphate (PI(4,5)P₂). Membrane invaginations in vesicles were observed forming away from the leaflet with higher Ca²⁺ concentration, showing that Ca²⁺ induces negative curvature. We hypothesize that the negative curvature is produced by Ca²⁺-induced clustering of PS and PI(4,5)P₂. In support of this notion, we find that Ca²⁺-induced membrane deformation is stronger for membranes containing PI(4,5)P₂, which is known to more readily cluster in the presence of Ca²⁺. The observed Ca²⁺-induced membrane deformation is strongly influenced by Na⁺ ions. A high symmetric [Na⁺] across the membrane reduces Ca²⁺ binding by electrostatic shielding, inhibiting Ca²⁺-induced membrane deformation. An asymmetric [Na⁺] across the membrane, however, can either oppose or support Ca²⁺-induced deformation, depending on the direction of the gradient in [Na⁺]. At a sufficiently high asymmetric Na⁺ concentration it can impact membrane structure in the absence of Ca²⁺. We propose that Ca²⁺ works in concert with curvature generating proteins to modulate membrane curvature and shape transitions. This novel structural impact of Ca²⁺ could be important for Ca²⁺-dependent cellular processes that involve the creation of membrane curvature, including exocytosis, invadopodia, and cell motility.