Ryanodine receptor 1 (RYR1) is the sarcoplasmic reticulum (SR) Ca²⁺ release channel required for both skeletal muscle contraction and Ca²⁺ leak. Mutations in RYR1 cause malignant hyperthermia susceptibility (MHS) and enhanced sensitivity to heat stroke (ESHS), which can result in death due to excessive skeletal muscle thermogenesis upon exposure to volatile anesthetics or heat. Here, we investigated the molecular and physiological functions of phosphorylation of RYR1 at Ser²⁹⁰² by the kinase striated muscle preferentially expressed protein (SPEG). Muscle from SPEG-deficient mice expressing RYR1 with a Ser²⁹⁰²→Asp²⁹⁰² (S2902D) point mutation to mimic phosphorylation by SPEG showed decreased SR Ca²⁺ sparks. Muscle from mice homozygous for the S2902D point mutation had reduced SR Ca²⁺ transients and small changes in force generation but overall mild phenotypic changes. YS mice, which are heterozygous for a Tyr⁵²⁴→Ser⁵²⁴ point mutation in RYR1, show increased Ca²⁺ leak and are a model of MHS and ESHS. Crossing YS mice with S2902D mice led to decreased SR Ca²⁺ leak and desensitized the mice to both volatile anesthetics and heat. Thus, SPEG inhibits SR Ca²⁺ leak in skeletal muscle by phosphorylating Ser²⁹⁰² on RYR1, and mutation of Ser²⁹⁰² to Asp²⁹⁰² to mimic this phosphorylation event rescues YS mice from heat-induced death.