EF-hand proteins are ubiquitous in cell signaling. Parvalbumin (Parv), the archetypal EF-hand protein, is a high-affinity Ca²⁺ buffer in many biological systems. Given the centrality of Ca²⁺ signaling in health and disease, EF-hand motifs designed to have new biological activities may have widespread utility. Here, an EF-hand motif substitution that had been presumed to destroy EF-hand function, that of glutamine for glutamate at position 12 of the second cation binding loop domain of Parv (ParvE101Q), markedly inverted relative cation affinities: Mg²⁺ affinity increased, whereas Ca²⁺ affinity decreased, forming a new ultra-delayed Ca²⁺ buffer with favorable properties for promoting cardiac relaxation. In therapeutic testing, expression of ParvE101Q fully reversed the severe myocyte intrinsic contractile defect inherent to expression of native Parv and corrected abnormal myocardial relaxation in diastolic dysfunction disease models in vitro and in vivo. Strategic design of new EF-hand motif domains to modulate intracellular Ca²⁺ signaling could benefit many biological systems with abnormal Ca²⁺ handling, including the diseased heart.