Subjecting cardiomyocytes to mechanical stress or neurohumoral stimulation causes cardiac hypertrophy characterized in part by reactivation of the fetal cardiac gene program. Here we demonstrate a new common mechanism by which these stimuli are transduced to a signal activating the hypertrophic gene program. Mechanically stretching cardiomyocytes induced nuclear accumulation of myocardin-related transcription factor A (MRTF-A), a coactivator of serum response factor (SRF), in a Rho- and actin dynamics-dependent manner. Expression of brain natriuretic peptide (BNP) and other SRF-dependent fetal cardiac genes in response to acute mechanical stress was blunted in mice lacking MRTF-A. Hypertrophic responses to chronic pressure overload were also significantly attenuated in mice lacking MRTF-A. Mutation of a newly identified, conserved and functional SRF-binding site within the BNP promoter, or knockdown of MRTF-A, reduced the responsiveness of the BNP promoter to mechanical stretch. Nuclear translocation of MRTF-A was also involved in endothelin-1- and angiotensin-II-induced activation of the BNP promoter. Moreover, mice lacking MRTF-A showed significantly weaker hypertrophic responses to chronic angiotensin II infusion than wild-type mice. Collectively, these findings point to nuclear translocation of MRTF-A as a novel signaling mechanism mediating both mechanical stretch- and neurohumoral stimulation-induced BNP gene expression and hypertrophic responses in cardiac myocytes.