In this paper, we establish a multiplicative equivalence between two multiplicative algebraic K-theory constructions, Elmendorf and Mandell's version of Segal's K-theory and Blumberg and Mandell's version of Waldhausen's S• construction. This equivalence implies that the ring spectra, algebra spectra, and module spectra constructed via these two classical algebraic K-theory functors are equivalent as ring, algebra or module spectra, respectively. It also allows for comparisions of spectrally enriched categories constructed via these definitions of Ktheory. As both the Elmendorf-Mandell and Blumberg-Mandell multiplicative versions of K-theory encode their multiplicativity in the language of multicategories, our main theorem is that there is multinatural transformation relating these two symmetric multifunctors that lifts the classical functor from Segal's to Waldhausen's construction. Along the way, we provide a slight generalization of the Elmendorf-Mandell construction to symmetric monoidal categories.Second, we focus on K-theory as a construction on Waldhausen categories, rather than any of the classes of infinity categories. Since the classical Waldhausen and Segal constructions apply only to more specific types of input, their uniqueness is not addressed by any of the uniqueness or universality results above. The present paper provides this kind of multiplicative comparison between the classical versions of algebraic K-theory.