Neuromuscular diseases such as amyotrophic lateral sclerosis and sarcopenia cause muscle atrophy, which preferentially affects fast-twitch glycolytic myofibers. The mechanisms underlying the susceptibility of fast myofibers to disease remain unclear. To investigate this, we analyzed the transcriptional profiles of myonuclei from denervated muscle fibers. We found that the fast muscle gene program and the transcription factor Maf were repressed upon denervation. Overexpression of Maf in mice prevented loss of muscle mass caused by denervation by repressing atrophic genes and restoring fast gene expression. Similar repression of fast genes and Maf was observed in muscles from mice and humans with amyotrophic lateral sclerosis. Notably, Maf overexpression in human skeletal muscle cells in vitro prevented muscle atrophy and activated the expression of fast muscle genes. Our findings highlight a key role for Maf in maintaining muscle mass and could offer a promising therapeutic strategy to preserve muscle function during disease, aging, and injury.