The heart is formed from cardiogenic progenitors expressing the transcription factors Nkx2-5 and Isl1 (refs 1 and 2). These multipotent progenitors give rise to cardiomyocyte, smooth muscle and endothelial cells, the major lineages of the mature heart. Here we identify a novel cardiogenic precursor marked by expression of the transcription factor Wt1 and located within the epicardium-an epithelial sheet overlying the heart. During normal murine heart development, a subset of these Wt1(+) precursors differentiated into fully functional cardiomyocytes. Wt1(+) proepicardial cells arose from progenitors that express Nkx2-5 and Isl1, suggesting that they share a developmental origin with multipotent Nkx2-5(+) and Isl1(+) progenitors. These results identify Wt1(+) epicardial cells as previously unrecognized cardiomyocyte progenitors, and lay the foundation for future efforts to harness the cardiogenic potential of these progenitors for cardiac regeneration and repair.