Despite extensive research on the taxonomy and phylogeny of the subfamily Aphidiinae Haliday, 1833, certain questions about the relationships between genera remain unresolved. Genera Trioxys Haliday, 1833 and Binodoxys Mackauer, 1960 are considered closely related, based on morphological and molecular analyses. However, recent studies suggest there is a need for a taxonomic revision of the two genera, since molecular data does not support monophyly of the two groups when a larger number of species is used in the analysis. We examine those relationships using molecular data and including a new species we describe in the present study. Trioxys ulmi Čkrkić & Tomanović, n. sp. is a parasitoid of the Japanese elm aphid (Tinocallis takachihoensis Higuchi, 1972) on elm hybrids (Ulmus x hollandica Mill.). Despite its probable Asian origin, this species has gone undescribed until its accidental introduction to Europe, highlighting the importance of continued research efforts.