A trisection of a smooth, closed, oriented 4-manifold is a decomposition into three 4-dimensional 1-handlebodies meeting pairwise in 3-dimensional 1handlebodies, with triple intersection a closed surface. The fundamental groups of the surface, the 3-dimensional handlebodies, the 4-dimensional handlebodies, and the closed 4-manifold, with homomorphisms between them induced by inclusion, form a commutative diagram of epimorphisms, which we call a trisection of the 4-manifold group. A trisected 4-manifold thus gives a trisected group; here we show that every trisected group uniquely determines a trisected 4-manifold. Together with Gay and Kirby's existence and uniqueness theorem for 4-manifold trisections, this gives a bijection from group trisections modulo isomorphism and a certain stabilization operation to smooth, closed, connected, oriented 4-manifolds modulo diffeomorphism. As a consequence, smooth 4-manifold topology is, in principle, entirely group theoretic. For example, the smooth 4-dimensional Poincaré conjecture can be reformulated as a purely group theoretic statement. * 57M05; 20F05 Let g and k be integers with g ≥ k ≥ 0. We fix the following groups, described explicitly by presentations:standard genus g surface group with standard labelled generators. We identify this in the obvious way with π 1 (# g S 1 × S 1 , * ).• H 0 = {1} and, for g > 0, H g = x 1 , . . . , x g , i.e. a free group of rank g with g labelled generators. We identify this in the obvious way with π 1 ( g S 1 × B 2 , * ). Note that, if g < g , then H g ⊂ H g .