The plant bug genus Semium Reuter, previously containing six species, is reviewed and the new Mexican species Semium albocuneatum from Puebla and S. cervantesi from Guerrero, and S. texanum from western Texas are described. Previously known species are diagnosed and redescribed, a color dorsal illustration of S. albocuneatum, color dorsal images of all species, selected SEM images of S. hirtum, and illustrations of male genitalia are provided, and a revised key to the nine species of the genus is given.