Exercise induces physiological cardiac growth and protects the heart against pathological remodeling. Recent work suggests exercise also enhances the heart’s capacity for repair, which could be important for regenerative therapies. While microRNAs are important in certain cardiac pathologies, less is known about their functional roles in exercise-induced cardiac phenotypes. We profiled cardiac microRNA expression in two distinct models of exercise and found microRNA-222 (miR-222) was upregulated in both. Downstream miR-222 targets modulating cardiomyocyte phenotypes were identified, including HIPK1 and HMBOX1. Inhibition of miR-222 in vivo completely blocked cardiac and cardiomyocyte growth in response to exercise while reducing markers of cardiomyocyte proliferation. Importantly, mice with inducible cardiomyocyte miR-222 expression were resistant to adverse cardiac remodeling and dysfunction after ischemic injury. These studies implicate miR-222 as necessary for exercise-induced cardiomyocyte growth and proliferation in the adult mammalian heart and show that it is sufficient to protect the heart against adverse remodeling. Display omitted •MicroRNAs concordantly regulated by distinct forms of exercise were identified•miR-222 induces cellular hypertrophy and proliferation in vitro via distinct targets•Inhibition of miR-222 in vivo completely blocks exercise-induced cardiac growth•Cardiac expression of miR-222 reduces adverse remodeling after ischemic injury Exercise has many cardiovascular benefits that are only partly understood. Liu et al. identify miR-222, together with its targets p27, HIPK1, and HMBOX1, as an important mediator of exercise-induced cardiac growth. Cardiac expression of miR-222 also protects against adverse remodeling and cardiac dysfunction after ischemic injury.