Nearctic Stylogaster Macquart (Diptera: Conopidae) species concepts are revised. We provide redescriptions of S. biannulata (Say) and S. neglecta Williston, description of S. beresfordi new species, an illustrated key to species, range maps, and ecological data. Stylogaster beresfordi ranges from southern Mexico to southern New Mexico, United States of America. DNA barcode data are provided with neighbour-joining and Bayesian analyses and relationships of the three species are discussed. We hypothesise that S. neglecta and S. beresfordi are sister taxa, while S. biannulata is in a different clade.