Although cytoskeletal mutations are known causes of genetically based forms of dilated cardiomyopathy, the pathways that link these defects with cardiomyopathy are unclear. Here we report that the α-actinin–associated LIM protein (ALP; Alp in mice) has an essential role in the embryonic development of the right ventricular (RV) chamber during its exposure to high biomechanical workloads in utero. Disruption of the gene encoding Alp (Alp) is associated with RV chamber dilation and dysfunction, directly implicating α-actinin–associated proteins in the onset of cardiomyopathy. In vitro assays showed that Alp directly enhances the capacity of α-actinin to cross-link actin filaments, indicating that the loss of Alp function contributes to destabilization of actin anchorage sites in cardiac muscle. Alp also colocalizes at the intercalated disc with α-actinin and γ-catenin, the latter being a known disease gene for human RV dysplasia. Taken together, these studies point to a novel developmental pathway for RV dilated cardiomyopathy via instability of α-actinin complexes.