The regenerative capacity of the adult mammalian heart is limited, because of the reduced ability of cardiomyocytes to progress through mitosis(1). Endogenous cardiomyocytes have regenerative capacity at birth but this capacity is lost postnatally, with subsequent organ growth occurring through cardiomyocyte hypertrophy(2,3). The Hippo pathway, a conserved kinase cascade, inhibits cardiomyocyte proliferation in the developing heart to control heart size and prevents regeneration in the adult heart(4,5). The dystrophingly-coprotein complex (DGC), a multicomponent transmembrane complex linking the actin cytoskeleton to extracellular matrix, is essential for cardiomyocyte homeostasis. DGC deficiency in humans results in muscular dystrophy, including the lethal Duchenne muscular dystrophy. Here we show that the DGC component dystroglycan 1 (Dag1) directly binds to the Hippo pathway effector Yap to inhibit cardiomyocyte proliferation in mice. The Yap-Dag1 interaction was enhanced by Hippo-induced Yap phosphorylation, revealing a connection between Hippo pathway function and the DGC. After injury, Hippo-deficient postnatal mouse hearts maintained organ size control by repairing the defect with correct dimensions, whereas postnatal hearts deficient in both Hippo and the DGC showed cardiomyocyte overproliferation at the injury site. In the hearts of mature Mdx mice (which have a point mutation in Dmd)-a model of Duchenne muscular dystrophy-Hippo deficiency protected against overload-induced heart failure.