We define a D = 26 + 1 Monstrous, purely bosonic M-theory, whose massless spectrum, of dimension 196, 884, is acted upon by the Monster group. Upon reduction to D = 25 + 1, this gives rise to a plethora of non-supersymmetric, gravito-dilatonic theories, whose spectrum irreducibly splits under the Monster as 196, 884 = 1 ⊕ 196, 883, where the singlet is identified with the dilaton, and 196, 883 denotes the smallest non-trivial representation of the Monster. This clarifies the definition of the Monster as the automorphism of the Griess algebra, by showing that such an algebra is not merely a sum of unrelated spaces, but rather an algebra of massless states for a Monstrous M-theory, which includes Horowitz and Susskind's bosonic M-theory as a subsector. Remarkably, a certain subsector of Monstrous M-theory, when coupled to a Rarita-Schwinger massless field in 26 + 1, exhibits the same number of bosonic and fermionic degrees of freedom.