G-protein–coupled receptor (GPCR) agonists are well-known inducers of cardiac hypertrophy. We found that the shedding of heparin-binding epidermal growth factor (HB-EGF) resulting from metalloproteinase activation and subsequent transactivation of the epidermal growth factor receptor occurred when cardiomyocytes were stimulated by GPCR agonists, leading to cardiac hypertrophy. A new inhibitor of HB-EGF shedding, KB-R7785, blocked this signaling. We cloned a disintegrin and metalloprotease 12 (ADAM12) as a specific enzyme to shed HB-EGF in the heart and found that dominant-negative expression of ADAM12 abrogated this signaling. KB-R7785 bound directly to ADAM12, suggesting that inhibition of ADAM12 blocked the shedding of HB-EGF. In mice with cardiac hypertrophy, KB-R7785 inhibited the shedding of HB-EGF and attenuated hypertrophic changes. These data suggest that shedding of HB-EGF by ADAM12 plays an important role in cardiac hypertrophy, and that inhibition of HB-EGF shedding could be a potent therapeutic strategy for cardiac hypertrophy.