A new betaine salt corresponding to the formula ##STR1## is disclosed. It is obtained in pure crystalline form by preparing a starting aqueous solution containing the ions ##STR2## Na.sup.+ and Cl.sup.- whose composition, as illustrated in a ternary diagram of betaine--NaCl--water shown in the drawing, is situated in an area between two lines diverging from a common point T and passing through the points I and II, adding to the starting solution betaine, sodium chloride and water in molar proportions of 1:1:2 or betaine hydrochloride, sodium hydroxide and water in molar proportions of 1:1:1, and thereafter separating the double betaine salt by crystallization at a temperature below 50.degree. C. The double betaine salt is useful as a seasoning.