Balanced words on a finite alphabet are those words in which every letter of the alphabet occurs the same number of times. The notion of descents and major index extends in a natural way to words. It is known that the bivariate generating polynomials for descents and major index over balanced words on the alphabet $[k]$ with $n$ occurrences each is palindromic, but a bijective proof has been missing even for balanced binary words. We give an explicit bijection proving this result. For permutations (which are also balanced), our bijection is different from the complementation map. We also show that for balanced binary words, this bijection simultaneously flips the ascent and comajor index as well.