A new species of Thecomyia Perty, 1833, T. diederiki Mortelmans n. sp., is described from French Guiana, with diagnostic characters being the absence of anteromedial setae on the midfemur; an ill-defined, tripartite, greyish mesonotal stripe; absence of upper fronto-orbital bristles; completely yellow mid- and hind femora; typical distiphallus bearing two long, curved acrophalli; absence of male anterior gonostyli; and large, triangular posterior gonostyli. All specimens were collected during the Mitaraka (French Guiana) 2015 survey that investigated an array of habitat types; the new species was encountered only in swamp forests. This discovery raises the number of species in this exclusively Neotropical genus to 13. An update of the relevant couplets in the key to Thecomyia in Marinoni et al. (2003) is presented to include the new species. New records of other species of Thecomyia from Costa Rica, French Guiana, and Brazil are given, with first records of Thecomyia lateralis (Walker, 1858) from Costa Rica, and first records of T. diederiki Mortelmans n. sp. and T. longicornis Perty, 1833 from French Guiana.
Acalyptratae, Sciomyzoidea, French Guiana, Costa Rica, geographical distribution, neotropical, snail-killing flies, survey, new species.