Congenital heart diseases are the most commonly observed human birth defects and are the leading cause of infant morbidity and mortality. Accumulating evidence indicates that transforming growth factor-β/bone morphogenetic protein signaling pathways play critical roles during cardiogenesis. Smad4 encodes the only common Smad protein in mammals, which is a critical nuclear mediator of transforming growth factor-β/bone morphogenetic protein signaling. The aim of this work was to investigate the roles of Smad4 during heart development. To overcome the early embryonic lethality of Smad4 −/− mice, we specifically disrupted Smad4 in the myocardium using a Cre/loxP system. We show that myocardial-specific inactivation of Smad4 caused heart failure and embryonic lethality at midgestation. Histological analysis revealed that mutant mice displayed a hypocellular myocardial wall defect, which is likely the primary cause for heart failure. Both decreased cell proliferation and increased apoptosis contributed to the myocardial wall defect in mutant mice. Data presented in this article contradict a previous report showing that Smad4 is dispensable for heart development. Our further molecular characterization showed that expression of Nmyc and its downstream targets, including cyclin D1, cyclin D2, and Id2, were downregulated in mutant embryos. Reporter analysis indicated that the transcriptional activity of the 351-bp Nmyc promoter can be positively regulated by bone morphogenetic protein stimulation and negatively regulated by transforming growth factor-β stimulation. Chromatin immunoprecipitation analysis revealed that the Nmyc promoter can form a complex with Smad4, suggesting that Nmyc is a direct downstream target of Smad4. In conclusion, this study provides the first mouse model showing that Smad4 plays essential roles during cardiogenesis.