The Eucelatoria ferox group is a lineage of 26 species of the New World genus Eucelatoria Townsend (Tachinidae) in which the female bears a distinctive sword-like piercer. The E. ferox group is revised and illustrated in detail, including nine species redescriptions and seventeen new species. This work also includes three new species synonyms and one new genus synonym. Three morphology-based subgroups are recognized and supported by genetic and host evidence as well as by geographic patterns. The E. ferox subgroup contains species with only two dorsal thoracic vittae on both the presutural and postsutural areas, including Eucelatoria ferox (Townsend), E. huitepecensis sp. nov., and E. inclani sp. nov. The E. gladiatrix subgroup contains those species with four dorsal thoracic vittae on both the presutural and postsutural areas, including Eucelatoria aurata (Townsend), E. auriceps (Aldrich), E. borealis sp. nov., E. charapensis (Townsend), E. crambivora sp. nov., E. falcata sp. nov., E. fordlandia sp. nov., E. gladiatrix (Townsend), E. jorgecortesi sp. nov., E. luctuosa (Wulp), E. rivalis (Reinhard), E. sabroskyi sp. nov., E. strigata (Wulp), E. texana (Reinhard), E. woodorum sp. nov., and E. yanayacu sp. nov. The E. kopis subgroup contains species with a single large dorsal thoracic vitta on the postsutural area and usually two vittae on the presutural area, including Eucelatoria gustavogutierrezi sp. nov., E. hafelei sp. nov., E. kopis sp. nov., E. makhaira sp. nov., E. ritavargasae sp. nov., E. sica sp. nov., and E. tenebrionis sp. nov. The following new synonymies are proposed: Proroglutea Townsend under Eucelatoria Townsend, syn. nov.; Eucelatoria fasciata (Townsend) and E. minima (Wulp) under E. strigata (Wulp), syn. nov.; Proroglutea pilligera Townsend under Eucelatoria gladiatrix (Townsend), syn. nov. Additionally, Telothyria lugens Wulp and Exorista obscurata Wulp are removed from synonymy with Eucelatoria luctuosa (Wulp) and resurrected as Eucelatoria lugens (Wulp) and E. obscurata (Wulp). The E. gladiatrix subgroup appears to be specialized on Crambidae (Lepidoptera) caterpillars, while the E. kopis subgroup parasitizes a variety of Lepidoptera families; host use for the E. ferox subgroup is unknown. The characteristic “leaf-rolling” habit of host families and visual evidence from rearing records suggests that the long piercers are used to penetrate leaf tissue for oviposition. Species of the E. ferox group are distributed throughout the Western Hemisphere from southern Canada to northern Argentina, with the highest species diversity and likely center of origin in the Central and South American tropics.