Significance Myostatin, via activation of the Smad2/3 pathway, has long been recognized as the body’s major negative regulator of skeletal muscle mass. In this study, however, we demonstrate that other TGF-β proteins, particularly activin A and activin B, act in concert with myostatin to repress muscle growth. Preventing activin and myostatin signaling in the tibialis anterior muscles of mice resulted in massive hypertrophy (>150%), which was dependent upon both the complete inhibition of the Smad2/3 pathway and activation of the parallel bone morphogenetic protein (BMP)/Smad1/5 axis. Using this approach in models of muscular dystrophy and cancer cachexia increased muscle mass or prevented muscle wasting, respectively, highlighting the potential therapeutic advantages of complete inhibition of Smad2/3 ligand activity in skeletal muscle.