In a test of L-methionine to suppress tomato powdery mildew caused by Oidium neolycopersici, tomato leaves were sprayed with 100 μg/mL L-methionine solution 2 days before O. neolycopersici conidia were brushed onto leaves. Disease severity was scored at 7 days post-inoculation (dpi). Compared with the severity on untreated controls, severity on L-methionine-treated tomato plants was reduced. Light microscopic observation showed fewer conidiophores, and haustoria developed on L-methionine-treated plant leaves than on control leaves. When relative expression of PR genes was analyzed by RT-PCR in tomato leaves at 2 dpi, expression was higher in L-methionine-treated plants than in the controls. These results suggest that L-methionine treatment of tomato plants induced resistance against O. neolycopersici and decreased the severity of powdery mildew.