Cardiac ankyrin repeat protein (CARP) is a cardiac-specific stress-response protein which exerts diverse effects to modulate cardiac remodeling in response to pathological stimuli. We examined the role of CARP in postnatal cardiac development and function under basal conditions in mice. Transgenic mice that selectively overexpressed CARP in heart (CARP Tg) exhibited dilated cardiac chambers, impaired heart function, and cardiac fibrosis as assessed by echocardiography and histological staining. Furthermore, the mice had a shorter lifespan and reduced survival rate in response to ischemic acute myocardial infarction. Immunofluorescence demonstrated the overexpressed CARP protein was predominantly accumulated in the nuclei of cardiomyocytes. Microarray analysis revealed that the nuclear localization of CARP was associated with the suppression of calcium-handling proteins. In vitro experiments revealed that CARP overexpression resulted in decreased cell contraction and calcium transient. In post-mortem cardiac specimens from patients with dilated cardiomyopathy and end-stage heart failure, CARP was significantly increased. Taken together, our data identified CARP as a crucial contributor in dilated cardiomyopathy and heart failure which was associated with its regulation of calcium-handling proteins.