To provide a method for producing a 3-(9-fluorenylmethylthio)fatty acid in high yield.

The method for producing the (9-fluorenylmethylthio)fatty acid comprises conducting a reaction of the corresponding mercaptofatty acid with a compound represented by formula (IIa): Fm-R1 (wherein, Fm is 9-fluorenylmethyl; R1 is a chlorine atom, a bromine atom, an iodine atom, p-toluenesulfonyloxy, methanesulfonyloxy or succinimidyloxy) or a compound represented by formula (IIb): Fmoc-R2 (wherein, Fmoc is 9-fluorenylmethoxycarbonyl; R2 is succinimidyloxy, benzotriazol-1-yloxy or 5-norbornene-2,3-dicarboxyimidyloxy) in the presence of an inorganic base.