Abstract: The invention provides a novel crystalline potassium salt of .beta.-nicotinamide adenine dinucleotide phosphoric acid (.beta.-NADP-K), having a primitively orthorhombic crystal system, which salt is more pure and more stable than the conventional amorphous form, and is advantageously not hygroscopic. The novel salt is produced by treating a dilute aqueous solution of the potassium salt of .beta.-nicotinamide adenine dinucleotide phosphoric acid with a hydrophilic water soluble organic solvent at a pH value of 2 to 5 until turbidity sets in, permitting crystallization and then separating the crystals.