Several pathophysiological abnormalities, including a sedentary lifestyle, chronic diseases, and oxidative stress, can contribute to muscle atrophy triggered by an imbalance in muscle protein synthesis and degradation. Resolving muscle atrophy is a critical issue as it can reduce the quality of life. Here, one of the promising functional food factors, diosgenin (a steroidal sapogenin) showed strong preventive activities against dexamethasone (Dex)-induced muscle atrophy, as determined by the expression levels and morphology of the myosin heavy chain in C2C12 myotubes. Diosgenin inhibited protein expressions of Dex-induced skeletal muscle-specific ubiquitin ligase, including muscle RING finger 1 (MuRF1) and casitas B-lineage lymphoma protooncogene b (Cbl-b) but not atrogin-1. Diosgenin ameliorated Dex-induced declines of Akt phosphorylation at Ser473 and FoxO3a phosphorylation at Ser253, which probably at least partially contributed to the suppression of MuRF1, Cbl-b, and atrogin-1 gene expression. Additionally, diosgenin inhibited Dex-induced nuclear translocation of the glucocorticoid receptor (GR), diosgenin therefore may competitively inhibit the interaction between Dex and GR. These findings suggest that diosgenin is an effective functional food for preventing glucocorticoid-induced skeletal muscle atrophy.