The possibility of the release of smallpox virus into a predominantly nonimmunized population highlights the importance of understanding poxvirus biology. Poxviruses encode a conserved pathway that is required to oxidize disulfide bonds in nascent viral proteins that fold in the reducing environment of the eukaryotic host cytoplasm. We present the structure of the last enzyme of the vaccinia virus pathway, G4, which is almost identical in smallpox virus. G4 catalyzes the formation of disulfide bonds in proteins that are critical for virus maturation and host cell infection. G4 contains a thioredoxin fold and a Cys-X-X-Cys active site. In solution, G4 monomers and dimers are observed. In the crystal, G4 is found as a dimer that buries 4,500 A(2) in the interface and occludes the active site, which could protect the reactive disulfide from reduction in the cytoplasm. The structure serves as a model for drug design targeting viral disulfide bond formation.