To initiate an efficient primary infection, it is important for baculovirus virions to penetrate through the peritrophic membrane (PM) of the host insect. It is frequently reported that enhancins of baculoviruses significantly enhance viral infection by degrading the various protein components of PMs. However, not all baculoviruses encode enhancins. GP37s of baculoviruses share high amino acid identity with fusolins, synergistic factors found in entomopoxviruses. In this study, a truncated Cydia pomonella granulovirus GP37 was expressed in Escherichia coli. The expressed GP37 effectively bound to chitin, and binding occurred predominantly within 3 h. GP37 altered the protein profiles of Spodoptera exigua PMs, from which a 50-kDa protein was dissociated. Droplet-feeding bioassays indicated that GP37 significantly enhanced the infectivity of nucleopolyhedroviruses (NPVs) and the lethality of Bacillus thuringiensis (Bt) in S. exigua larvae. This is the first demonstration of the enhancement of NPVs and Bt infection by a baculovirus GP37.