A new species of Mydrosomella Michener, 1986 - M. caatinguensis Ferrari, Arantes et Silveira sp. nov. is described and illustrated. The bees were collected in the municipalities of Remígio and Sumé, in the state of Paraíba, and Canindé de São Francisco, state of Sergipe, all in the semiarid domain of Caatinga, in northeastern Brazil. The type series of the species includes five females collected in Malaise traps and one male captured on flowers of Waltheria indica Linnaeus (Malvaceae). Mydrosomella caatinguensis sp. nov. can be differentiated from the other known species in the genus by the mostly yellow pilosity, single preapical tooth on the female mandible, enlarged ocelli, and absence of arolia. This is the third species described in the genus and the only one recorded in a tropical region. An identification key to all Mydrosomella species is given.