Ten species of the genus Tethina Haliday from western North America are reviewed with an emphasis on structures of the male terminalia, which are illustrated. One new species (type locality in parenthesis) is described: Tethina sasakawai (California. San Diego: Camp Pendleton; 33°13.3′N, 117°24.5′W, beach). Tethina thula Sasakawa, which was described from specimens collected in Japan, is reported from Alaska. Four new synonymies are proposed: Rhicnoessa denudata Melander and Rhicnoessa lavendula Melander  =  Rhicnoessa angustipennis Melander; Tethina woodi Foster and Tethina steyskali Foster  =  Rhicnoessa milichioides Melander. Lectotypes are designated for the following five species' names: Tethina angustifrons Melander, Rhicnoessa angustipennis Melander, R. denudata Melander, R. lavendula Melander, and R. horripilans Melander. All species for which males are known are illustrated and distribution maps are provided.