Enterovirus infection can cause severe cardiomyopathy in humans. The virus-encoded 2A protease is known to cleave the cytoskeletal protein dystrophin. It is unclear, however, whether cardiomyopathy results from the loss of dystrophin or is due to the emergence of a dominant-negative dystrophin cleavage product. We show for the first time that the 2A protease–mediated carboxyl-terminal dystrophin cleavage fragment (CtermDys) is sufficient to cause marked dystrophic cardiomyopathy. The sarcolemma-localized CtermDys fragment caused myocardial fibrosis, heightened susceptibility to myocardial ischemic injury, and increased mortality during cardiac stress testing in vivo. CtermDys cardiomyopathy was more severe than in hearts completely lacking dystrophin. In vivo titration of CtermDys peptide content revealed an inverse relationship between the decay of membrane-bound CtermDys and the restoration of full-length dystrophin at the sarcolemma, in support of a physiologically relevant loss of dystrophin function in this model. CtermDys gene titration and dystrophin replacement studies further established a target threshold of 50% membrane-bound intact dystrophin necessary to prevent mice from CtermDys cardiomyopathy. Conversely, the NtermDys fragment did not compete with dystrophin and had no pathological effect. Thus, CtermDys must be localized to the sarcolemma, with intact dystrophin <50% of normal levels, to exert dominant-negative peptide-dependent cardiomyopathy. These data support a two-hit dominant-negative disease mechanism where membrane-associated CtermDys severs the link to cortical actin and inhibits both full-length dystrophin and compensatory utrophin from binding at the membrane. Therefore, membrane-bound CtermDys is a new potential translational target for virus-mediated cardiomyopathy.