  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 $S_\bullet$ 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 $K$-theory. 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.
