Caffeic acid O-methyltransferase (COMT) methylates N-acetylserotonin into melatonin; that is, it has N-acetylserotonin O-methyltransferase (ASMT) activity. The ASMT activity of COMT was first detected in Arabidopsis thaliana COMT (AtCOMT). To confirm the ASMT activity of COMT in other plant species, we evaluated the ASMT activity of a COMT from rice (Oryza sativa) (OsCOMT). Purified recombinant OsCOMT protein from Escherichia coli was used to validate the high ASMT activity of OsCOMT, similar to that of AtCOMT. The Km and Vmax values for the ASMT activity of OsCOMT were 243 μm and 2,400 pmol/min/mg protein, which were similar to those of AtCOMT. Similar to AtCOMT, OsCOMT was localized in the cytoplasm. In vitro ASMT activity was significantly inhibited by either caffeic acid or quercetin in a dose-dependent manner. Analogously, in vivo production of melatonin was significantly inhibited by quercetin in 4-week-old detached rice leaves, suggestive of a positive role of COMT in melatonin biosynthesis in plants.