A systematic revision of the genus Thinusa is presented. Thinusa Casey is redescribed. Thinusa fletcheri Casey and T. maritima (Casey) are redescribed. Lectotype and paralectotypes are designated for T. divergens, T. nigra, T. robustula, T. fletcheri, T. obscura and T. maritima from Casey's syntype series deposited in the NMNH. A key is provided for separation of the known species of Thinusa and illustrations of diagnostic features are presented. Based on maxilla with membranous galea, labium with bifid ligula, distributional pattern of pores on mandibles and lacinia, and anterior margin of abdominal tergites III-VI deeply and broadly V-shaped, and athetine bridge, Thinusa is hypothesized to be related to the tribe Athetini rather than the tribe Phytosini. Phylogenetic relationships of the Thinusa are discussed.