Abnormalities in intracellular Ca^2 + signaling have been proposed to play an essential role in the pathophysiology of atrial arrhythmias. However, a direct observation of intracellular Ca^2 + in atrial myocytes during atrial arrhythmias is lacking. Here, we have developed an ex vivo model of simultaneous Ca^2 + imaging and electrocardiographic recording in cardiac atria. Using this system we were able to record atrial arrhythmic intracellular Ca^2 + activities. Our results indicate that atrial arrhythmias can be tightly linked to intracellular Ca^2 + waves and Ca^2 + alternans. Moreover, we applied this strategy to analyze Ca^2 + signals in the hearts of WT and knock-in mice harboring a ‘leaky’ type 2 ryanodine receptor (RyR2-R2474S). We showed that sarcoplasmic reticulum (SR) Ca^2 + leak increases the susceptibility to Ca^2 + alternans and Ca^2 + waves increasing the incidence of atrial arrhythmias. Reduction of SR Ca^2 + leak via RyR2 by acute treatment with S107 reduced both Ca^2 + alternans and Ca^2 + waves, and prevented atrial arrhythmias.