Despite the importance of cardiomyocyte proliferation in cardiac development and regeneration, the mechanisms that promote cardiomyocyte cell cycle remain incompletely understood. RE1 silencing transcription factor (REST) is a transcriptional repressor of neuronal genes. Here we show that REST also regulates the cardiomyocyte cell cycle. REST binds and represses the cell cycle inhibitor gene p21 and is required for mouse cardiac development and regeneration. Rest deletion de-represses p21 and inhibits the cardiomyocyte cell cycle and proliferation in embryonic or regenerating hearts. By contrast, REST overexpression in cultured cardiomyocytes represses p21 and increases proliferation. We further show that p21 knockout rescues cardiomyocyte cell cycle and proliferation defects resulting from Rest deletion. Our study reveals a REST-p21 regulatory axis as a mechanism for cell cycle progression in cardiomyocytes, which might be exploited therapeutically to enhance cardiac regeneration.