Satellite cells play a critical role in skeletal muscle regeneration in response to injury. Notch signaling is vital for satellite cell activation and myogenic precursor cell expansion but inhibits myogenic differentiation. Thus, precise spatial and temporal regulation of Notch activity is necessary for efficient muscle regeneration. We report that the basic helix-loop-helix transcription factor Stra13 modulates Notch signaling in regenerating muscle. Upon injury, Stra13(-/-) mice exhibit increased cellular proliferation, elevated Notch signaling, a striking regeneration defect characterized by degenerated myotubes, increased mononuclear cells, and fibrosis. Stra13(-/-) primary myoblasts also exhibit enhanced Notch activity, increased proliferation, and defective differentiation. Inhibition of Notch signaling ex vivo and in vivo ameliorates the phenotype of Stra13(-/-) mutants. We demonstrate in vitro that Stra13 antagonizes Notch activity and reverses the Notch-imposed inhibition of myogenesis. Thus, Stra13 plays an important role in postnatal myogenesis by attenuating Notch signaling to reduce myoblast proliferation and promote myogenic differentiation.