A new species, Xestomyza stuckenbergi sp. n., is described from the Western Cape Province, South Africa. The new species differs from the only previously described species of this genus, Xestomyza lugubris Wiedemann, 1820, being smaller and entirely covered in white pubescence and also having differences in the male terminalia. A third species of this genus is briefly described, but because there is only one female specimen available it is not formally named. Identification keys are provided to genera of the subfamily Xestomyzinae and to species of Xestomyza Wiedemann.