Abstract: The present invention relates to a novel crystalline thio-NADP potassium salt, which is produced by preparing thionicotinamide adenine dinucleotide phosphate (thio-NADP) into the potassium salt thereof and thereafter crystallizing the salt. Conventionally known non-crystalline thio-NADP sodium salt has a strong hygroscopic property to deliquesce on contact with air, of which the storage is therefore difficult. The crystalline thio-NADP potassium salt of the present invention has no hygroscopic property so that it does not deliquesce on contact with air, which can be used as research reagents, diagnostic agents and the like for a long period of time and which can be stored for a long period of time.