The West-Palaearctic species of the Hercostomus species-group III are reviewed, a species-list and a revised key to males is provided. Hercostomus angustus (Loew, 1857) is restored from synonymy with Hercostomus nanus (Macquart, 1827) based on differences in male genitalia. Hercostomus griseifrons Becker, 1910 is redescribed and a lectotype is designated. Hercostomus tonguci sp. n. is described from Turkey.