The NSm nonstructural protein of Rift Valley fever virus (family Bunyaviridae, genus Phlebovirus) has an antiapoptotic function and affects viral pathogenesis. We found that NSm integrates into the mitochondrial outer membrane and that the protein's N terminus is exposed to the cytoplasm. The C-terminal region of NSm, which contains a basic amino acid cluster and a putative transmembrane domain, targeted the protein to the mitochondrial outer membrane and exerted antiapoptotic function.