The North American genus Tidestromia (Amaranthaceae: Gomphrenoideae), with high levels of endemism in the Chihuahuan Desert, is subjected to phylogenetic analysis using 19 morphological characters. Species from genera most closely related to Tidestromia (Alternanthera, Froelichia, Gossypianthus, and Guilleminea), as well as the more distantly related genus Iresine, were used as outgroups. The monophyly of Tidestromia is well supported by four synapomorphies: alternate leaves, dichasial inflorescences, and pollen that is psilate (homoplasious) and with mesoporia that are narrowed distally. The single most parsimonious tree resolved T. valdesiana as sister to the rest of Tidestromia, which is apomorphically diagnosed by the presence of involucres. The annual habit supports a clade comprising T. carnosa, T. lanuginosa, and T. tenella, which is nested among the species with involucres. The phylogenetic relationships of the rest of the taxa remain unresolved. Traditional morphological evidence thus is important for circumscribing the genus Tidestromia, but shows limited utility for resolving species groups within this genus.