Fatty degeneration of skeletal muscle leads to muscle weakness and loss of function. Preventing fatty degeneration in skeletal muscle is important, but no drug has been used clinically. In this study, we performed drug repositioning using human platelet-derived growth factor receptor a (PDGFR alpha)-positive mesenchymal progenitors that have been proved to be an origin of ectopic adipocytes in skeletal muscle. We found that promethazine hydrochloride (PH) inhibits adipogenesis in a dose-dependent manner without cell toxicity. PH inhibited expression of adipogenic markers and also suppressed phosphorylation of cAMP response-element binding protein, which was reported to be a primary regulator of adipogenesis. We established a mouse model of tendon rupture with intramuscular fat deposition and confirmed that emerged ectopic adipocytes are derived from PDGFR alpha(+) cells using lineage tracing mice. When these injured mice were treated with PH, formation of ectopic adipocytes was suppressed significantly. Our results show that PH inhibits PDGFR alpha(+) mesenchymal progenitor-dependent ectopic adipogenesis in skeletal muscle and suggest that treatment with PH can be a promising approach to prevent fatty degeneration of skeletal muscle.