A new species of Horismenus Walker, H. liturgusae Hansson & Schoeninger (Hymenoptera: Eulophidae), is described from material reared from an ootheca of an unidentified mantid species of genus Liturgusa Saussure (Mantodea: Liturgusidae). The new species is compared to H. argus Hansson, a species it is very similar to. Horismenus liturgusae sp. n. is gregarious and 49 adult specimens developed in the ootheca. This is the first record of a Horismenus species parasitizing mantids, and the first record of a Horismenus species from an ootheca.