Trichospilus albiflagellatus Yang & Wang (Hymenoptera: Eulophidae), a gregarious pupal endo-parasitoid of the fall webworm, Hyphantria cunea (Drury) (Lepidoptera: Arctiidae) is described. The host is an invasive pest that has caused severe damage to forest and ornamental trees since it was first discovered in 1979 in China. The pest has spread to 10 provinces, municipalities, or both in China. The parasitoid described in this article has great potential as a biocontrol agent for the defoliator because a parasitism rate of 28.57% has been observed and this species has a relatively high female:male sex ratio (58.56:1). In addition, an average of 159 parasitoid adults (range 102–219) emerge from a single host pupa. Detailed photographs of the key characters of the male and female of the new species are provided. Type specimens were deposited in Insect Museum, Chinese Academy of Forestry, Beijing, China.