P. V. Chung showed that there are many multiplicative functions f which satisfy f (m 2 + n 2 ) = f (m 2 ) + f (n 2 ) for all positive integers m and n. In this article, we show that if more than 2 squares in the additive condition are involved, then such f is uniquely determined. That is, if a multiplicative function f satisfiesk ) for arbitrary positive integers a i , then f is the identity function. In this sense, we call the set of all posotive squares a k-additive uniqueness set for multiplicative functions.