The purpose of the present invention is to provide a method for producing selenoneine that allows production of selenoneine at higher yields as compared with a conventional technology, and, therefore, enables selenoneine production on an industrial scale. This purpose can be achieved by a method for producing selenoneine, comprising the step of applying histidine and a selenium compound to a transformant that has a gene encoding an enzyme of (1) below introduced therein and that can overexpress the introduced gene, to obtain selenoneine. (1) An enzyme that catalyzes a reaction in which hercynylselenocysteine is produced from histidine and selenocysteine in the presence of S-adenosylmethionine and iron (II).