Long noncoding RNAs (lncRNAs) are often expressed in a development-specific manner, yet little is known about their roles in lineage commitment. Here, we identified Braveheart (Bvht), a heart-associated lncRNA in mouse. Using multiple embryonic stem cell (ESC) differentiation strategies, we show that Bvht is required for progression of nascent mesoderm toward a cardiac fate. We find that Bvht is necessary for activation of a core cardiovascular gene network and functions upstream of mesoderm posterior 1 (MesP1), a master regulator of a common multipotent cardiovascular progenitor. We also show that Bvht interacts with SUZ12, a component of polycomb-repressive complex 2 (PRC2), during cardiomyocyte differentiation, suggesting that Bvht mediates epigenetic regulation of cardiac commitment. Finally, we demonstrate a role for Bvht in maintaining cardiac fate in neonatal cardiomyocytes. Together, our work provides evidence for a long noncoding RNA with critical roles in the establishment of the cardiovascular lineage during mammalian development. Display omitted ► Braveheart (Bvht) is a heart-associated long noncoding RNA in the mouse ► In embryonic stem cells, Bvht is necessary for cardiovascular lineage commitment ► Bvht functions upstream of MesP1 to regulate a core cardiac gene network ► Bvht interacts with PRC2 to mediate epigenetic activation of the cardiac program Braveheart (Bvht) is a heart-associated long noncoding RNA that is necessary for cardiovascular lineage commitment in embryonic stem cells. Its interaction with the polycomb complex suggests that it may regulate the cardiac program epigenetically.