The present disclosure relates to a method for synthesizing ergothioneine or one of the derivatives thereof of following formula (I): or a physiologically acceptable salt, a tautomer, a stereoisomer or a mixture of stereoisomers in all proportions thereof, from a compound of betaine type of following formula (II): or a physiologically acceptable salt, a tautomer, a stereoisomer or a mixture of stereoisomers in all proportions thereof, by cleavage reaction in the presence of a thiol, at a temperature above or equal to 60° C. The present disclosure also relates to compounds of formula (II) and the method of synthesis thereof.