Key points Phosphorylation at Ser‐2808 is suggested to result in RyR2 hyperactivity, i.e. ‘leakiness’, thus contributing to the pathology of cardiac diseases. We studied the effect of disabling phosphorylation at Ser‐2808 of RyR2 in a genetic model of Ca2+‐dependent cardiomyopathy, which was caused by leaky RyR2. RyR2 phosphorylation was high at Ser‐2808 in myocytes expressing wild‐type (WT) RyR2; protein phosphatase increased RyR2 leakiness in cells expressing WT, but not in mutant RyR2s with disabled Ser‐2808 phosphorylation sites. Rather than alleviating cardiac disease, ablation of the Ser‐2808 exacerbated the disease phenotype by reducing survival, impairing in vivo cardiac function and enhancing RyR2 Ca2+ leak and mitochondrial damage. These results suggest a novel mode of RyR2 regulation via dephosphorylation at Ser‐2808 in normal and diseased hearts. Abstract Phosphorylation of the cardiac ryanodine receptor (RyR2) by protein kinase A (PKA) at Ser‐2808 is suggested to mediate the physiological ‘fight or flight’ response and contribute to heart failure by rendering the sarcoplasmic reticulum (SR) leaky for Ca2+. In the present study, we examined the potential role of RyR2 phosphorylation at Ser‐2808 in the progression of Ca2+‐dependent cardiomyopathy (CCM) by using mice genetically modified to feature elevated SR Ca2+ leak while expressing RyR2s that cannot be phosphorylated at this site (S2808A). Surprisingly, rather than alleviating the disease phenotype, constitutive dephosphorylation of Ser‐2808 aggravated CCM as manifested by shortened survival, deteriorated in vivo cardiac function, exacerbated SR Ca2+ leak and mitochondrial injury. Notably, the deteriorations of cardiac function, myocyte Ca2+ handling, and mitochondria integrity were consistently worse in mice with heterozygous ablation of Ser‐2808 than in mice with complete ablation. Wild‐type (WT) and CCM myocytes expressing unmutated RyR2s exhibited a high level of baseline phosphorylation at Ser‐2808. Exposure of these CCM cells to protein phosphatase 1 caused a transitory increase in Ca2+ leak attributable to partial dephosphorylation of RyR2 tetramers at Ser‐2808 from more fully phosphorylated state. Thus, exacerbated Ca2+ leak through partially dephosphorylated RyR2s accounts for the prevalence of the disease phenotype in the heterozygous S2808A CCM mice. These results do not support the importance of RyR2 hyperphosphorylation in Ca2+‐dependent heart disease, and rather suggest roles for the opposite process, the RyR2 dephosphorylation at this residue in physiological and pathophysiological Ca2+ signalling.