Sumoylation is a posttranslational modification that regulates a wide spectrum of cellular activities. Cardiomyopathy is the leading cause of heart failure. Whether sumoylation, particularly SUMO-2/3 conjugation, is involved in cardiomyopathy has not been investigated. We report here that SUMO-2/3 conjugation was elevated in the human failing hearts, and we investigated the impact of increased SUMO-2 conjugation on heart function by using the gain-of-function approach in mice, in which cardiac specific expression of constitutively active SUMO-2 was governed by alpha myosin heavy chain promoter (MHC-SUMO-2 transgenic, SUMO-2-Tg). Four of five independent SUMO-2-Tg mouse lines exhibited cardiomyopathy with various severities, ranging from acute heart failure leading to early death to the development of chronic cardiomyopathy with aging. We further revealed that SUMO-2 directly regulated apoptotic process by at least partially targeting calpain 2 and its natural inhibitor calpastatin. SUMO conjugation to calpain 2 promoted its enzymatic activity, and SUMO attachment to calpastatin mainly promoted its turnover and altered its subcellular distribution. Thus, enhanced SUMO-2 conjugation led to increased apoptosis and played a pathogenic role in the development of cardiomyopathy and heart failure.