A new species of Trissolcus Ashmead (Hymenoptera: Scelionidae), Trissolcus halyomorphae Yang, sp. nov., is described from China. It is a solitary parasitoid in eggs of Halyomorpha halys (Stål) (Hemiptera: Pentatomidae), an economic pest that causes severe damage to pear, apple and other fruit and that also feeds on the leaves and stems of ornamental and forest trees. The new species is the principal natural enemy of the stink bug, with parasitism rates up to 70% and an average annual rate of 50%. Because of its high parasitism rates and other biological features, it has good potential as a biocontrol agent of the pest. Type specimens are deposited in Insect Museum, Chinese Academy of Forestry, Beijing, China.