Rift Valley fever virus (RVFV), like many other Bunyaviridae family members, is an emerging human and animal pathogen. Bunyaviruses have an outer lipid envelope bearing two glycoproteins, G(N) and G(C), required for cell entry. Bunyaviruses deliver their genome into the host-cell cytoplasm by fusing their envelope with an endosomal membrane. The molecular mechanism of this key entry step is unknown. The crystal structure of RVFV G(C) reveals a class II fusion protein architecture found previously in flaviviruses and alphaviruses. The structure identifies G(C) as the effector of membrane fusion and provides a direct view of the membrane anchor that initiates fusion. A structure of nonglycosylated G(C) reveals an extended conformation that may represent a fusion intermediate. Unanticipated similarities between G(C) and flavivirus envelope proteins reveal an evolutionary link between the two virus families and provide insights into the organization of G(C) in the outer shell of RVFV.