Fibroblast growth factor 21(FGF21) is a pivotal regulator of energy metabolism, which is currently being assessed as a potential drug target for the treatment of insulin‐resistant conditions. However, the cellular mechanisms by which FGF21 affects myogenesis remain unclear. In this study, we explored the function of FGF21 in myogenesis both in vitro and in vivo. Our experiments showed for the first time that FGF21 promotes myoblast differentiation and serves as a switch of molecular transformation from anaerobic myofibers to aerobic myofibers via the FGF21‐SIRT1‐AMPK‐PGC1α axis. Furthermore, we employed the Dual‐Luciferase Reporter Assay System and Electrophoretic Mobility Shift Assay (EMSA) and demonstrated that MYOD, a major myogenic transcription factor, binds directly to the promoter region of Fgf21, leading to the activation of Fgf21 expression in mouse C2C12 myoblasts. Our study revealed a novel mechanism of myogenesis and muscle fiber transformation and indicated that FGF21 serves as a vital regulator of muscle development and important contributor to the pathogenesis of myopathy. J. Cell. Physiol. 232: 1893–1906, 2017. © 2016 Wiley Periodicals, Inc. (1) The mouse FGF21 gene expression is positively regulated by the transcription factor MyoD. (2) FGF21 promotes myogenic differentiation and facilitates the switch of the muscle fiber type from anaerobic myofibers to aerobic myofibers via stimulation the FGF21‐SIRT1‐AMPK‐PGC1α axis. (3) The increased FGF21 protein induced by fasting is likely to change the expression of the PGC‐1α protein from a physiological to an abnormally excessive level via the activation of AMPK signaling, which results in muscle atrophy.