The pteromalid genus Trichilogaster is revised. All the species are associated with galls on native Australian acacias, with the exception of one from Arabia. Two species, namely T. acaciaelongifoliae and T. signiventris, have become established in South Africa following their introduction for biological control of Acacia longifolia and A. pycnantha. The following seven species are redescribed: T. acaciaelongifoliae (Froggatt), esculenta Ferrière, flavivena (Girault), maideni Froggatt, pendulae Mayr, signiventris (Girault) (all native to Australia) and arabica Ferrière from Saudi Arabia. Epiperilampus atra Girault, E. atricorpus Girault, E. atricorpus socratis Girault and E. significatus Girault are synonymized (syn. n.) with T. maideni. The following two new Australian species are described from both sexes: T. olgae and T. stefani. A key for the separation of the females of the nine known species of the genus is provided, as well as a list of the 13 species of known host plants, which include 12 species of Acacia and one of Paraserianthes.