Mutations in polycystin-2 (PC2) cause autosomal dominant polycystic kidney disease. A function for PC2 in the heart has not been described. Here, we show that PC2 coimmunoprecipitates with the cardiac ryanodine receptor (RyR2) from mouse heart. Biochemical assays showed that the N terminus of PC2 binds the RyR2, whereas the C terminus only binds to RyR2 in its open state. Lipid bilayer electrophysiological experiments indicated that the C terminus of PC2 functionally inhibited RyR2 channel activity in the presence of calcium (Ca²⁺). Pkd2^−/− cardiomyocytes had a higher frequency of spontaneous Ca²⁺ oscillations, reduced Ca²⁺ release from the sarcoplasmic reticulum stores, and reduced Ca²⁺ content compared with Pkd2^+/+ cardiomyocytes. In the presence of caffeine, Pkd2^−/− cardiomyocytes exhibited decreased peak fluorescence, a slower rate of rise, and a longer duration of Ca²⁺ transients compared with Pkd2^+/+. These data suggest that PC2 is important for regulation of RyR2 function and that loss of this regulation of RyR2, as occurs when PC2 is mutated, results in altered Ca²⁺ signaling in the heart.