Myostatin (Mstn) is predominantly expressed in skeletal muscles and plays important roles in regulating muscle growth and development, as well as fat deposition. Mstn‐knockout ( Mstn –/– ) mice exhibit increased muscle mass due to both hypertrophy and hyperplasia, and leaner body composition due to reduced fat mass. Here, we show that white adipose tissue (WAT) of Mstn –/– develops characteristics of brown adipose tissue (BAT) with dramatically increased expression of BAT signature genes, including Ucp1 and Pgc1α , and beige adipocyte markers Tmem26 and CD137. Strikingly, the observed browning phenotype is non‐cell autonomous and is instead driven by the newly defined myokine irisin (Fndc5) secreted from Mstn –/– skeletal muscle. Within the muscle, Mstn –/– leads to increased expression of AMPK and its phosphorylation, which subsequently activates PGC1α and Fndc5. Together, our study defines a paradigm of muscle‐fat crosstalk mediated by Fndc5, which is up‐regulated and secreted from muscle to induce beige cell markers and the browning of WAT in Mstn –/– mice. These results suggest that targeting muscle Mstn and its downstream signaling represents a therapeutic approach to treat obesity and type 2 diabetes.—Shan, T., Liang, X., Bi, P., Kuang, S. Myostatin knockout drives browning of white adipose tissue through activating the AMPK‐PGC1α‐Fndc5 pathway in muscle. FASEB J. 27, 1981–1989 (2013). www.fasebj.org