The new species Theocolax americanus McEwen, n. sp. (Hymenoptera, Pteromalidae, Cerocephalinae), is described from the United States of America. A description is provided with a diagnosis and comparison to similar species. A key to differentiate the Theocolax species recorded in the USA is included and notes given on new morphological features. Theocolax americanus is thought to attack the walnut twig beetle (Pityophthorus juglandis Blackman), which transmits thousand cankers disease. Sequencing of CO1 for the new species, other Theocolax species found in the U.S.A., Neocalosoter pityphthori Ashmead (Hymenoptera, Pteromalidae, Cerocephalinae), and P. juglandis is reported.