Recent reports suggest that the adult epicardium is a source of cardiac progenitor cells having the ability to undergo epithelial-to-mesenchymal transition (EMT) and predominantly differentiate into myofibroblasts, thereby contributing to fibrosis of the stressed myocardium. Islet-1 (Isl1) is a widely applied marker of progenitor cells, including the epicardial mesothelial cells (EMCs). However, little is known of the general biological function of Islet-1, let alone its role in EMT of EMCs. Using rat-derived adult EMC cultures we therefore investigated the role of Isl1 expression in both non-stimulated EMCs and during TGF-Β-induced EMT. We found that Isl1 had a dual role by promoting mesenchymal features in non-stimulated EMCs, while a loss of Isl1 associated with EMT acted as a negative modulator of EMT progression as assessed on phenotype. We furthermore found that the loss of Isl1 expression during EMT was, in addition to transcriptional regulation by Β-catenin, mediated through direct targeting by microRNA-31 (miR-31). Through manipulations of miR-31 bioactivity in EMCs, we thus report that miR-31 is a negative modulator of cardiac fibrogenic EMT, primarily via targeting Isl1. Our data show that Isl1 is a key regulatory molecule in adult cardiac EMT.