Skeletal muscle possesses a strong ability to regenerate following injury, a fact that has been largely attributed to satellite cells. Satellite cells are skeletal muscle stem cells located beneath the basal lamina of the myofiber, and are the principal cellular source of growth and regeneration in skeletal muscle. MicroRNAs (miRNAs) play key roles in modulating several cellular processes by targeting multiple mRNAs that comprise a single or multiple signaling pathway. Several miRNAs have been shown to regulate satellite cell activity, such as miRNA-489, which functions to maintain satellite cells in a quiescent state. Although muscle-specific miRNAs have been identified, many of the molecular mechanisms that regulate myogenesis that are regulated by miRNAs still remain unknown. In this study, we have shown that miR-128a is highly expressed in brain and skeletal muscle, and increases during myoblast differentiation. MiR-128a was found to regulate the target genes involved in insulin signaling, which include Insr (insulin receptor), Irs1 (insulin receptor substrate 1) and Pik3r1 (phosphatidylinositol 3-kinases regulatory 1) at both the mRNA and protein level. Overexpression of miR-128a in myoblasts inhibited cell proliferation by targeting IRS1. By contrast, inhibition of miR-128a induced myotube maturation and myofiber hypertrophy in vitro and in vivo. Moreover, our results demonstrate that miR-128a expression levels are negatively controlled by tumor necrosis factor α (TNF-α). TNF-α promoted myoblast proliferation and myotube hypertrophy by facilitating IRS1/Akt signaling via a direct decrease of miR-128a expression in both myoblasts and myotubes. In summary, we demonstrate that miR-128a regulates myoblast proliferation and myotube hypertrophy, and provides a novel mechanism through which IRS1-dependent insulin signaling is regulated in skeletal muscle.