A new species, Monokalliapseudes guianae, is described from French Guianese estuaries. It is distinguishable from its only congener most notably by lacking an exopodite on pereopod 1 and by the nature of the basal article of the uropod. The inner distal corner of the basal article is only slightly produced and lacks a rounded lobe. The bases of pereopods 2 and 3 lack numerous long setae. Sexual dimorphism is observed in the antennule, cheliped, and pereopod 1. Depending on size, males can exhibit two forms of chelae. A new diagnosis is presented for the genus Monokalliapseudes.