Using an atrial fibrillation (AF) model in spontaneously hypertensive rats (SHRs), we aimed to identify circulating miRNAs for AF diagnosis and prediction and to confirm the cardiac origin of these miRNAs. A total of 31 SHRs and 39 Wistar Kyoto (WKY) normotensive controls were randomized into six groups: young, adult, and aging SHR and WKY. Spontaneous AF burden and atrial and circulating levels of 11 miRNAs were quantified. Spontaneous AF was absent in all WKY rats. In the SHRs, AF episodes were observed in two adult animals and in all aging animals (13.6 ± 2.3 episodes/24 h). The atrial levels of five miRNAs were significantly higher in adult and aging SHRs compared to their WKY controls (all < 0.05). Of these, only the circulating levels of miR-328 were significantly higher in the aging SHRs vs. WKYs ( < 0.0001). Atrial miR-328 levels in the SHRs increased progressively with age ( < 0.001) and correlated with circulating miR-328 levels (r = 0.58; < 0.01). Among aging SHRs, atrial levels of miR-328 strongly correlated with AF burden (r = 0.79; < 0.01). These data suggest that the circulating level of miR-328 could emerge as a promising marker for both AF diagnosis and, if assessed dynamically, for AF prediction.