Phylogenetip relationships among the 25 species of the old-world genus Monsonia are explored by means of a cladistic analysis. After a detailed revision, 20 morphological and anatomical characters were selected, including some new ones from nectaries, androecium, and mericarps. Phylogenetic analysis yielded 9 most parsimonious trees. The strict consensus tree showed two major clades: one is formed by the 9 species of Monsonia sect. Monsonia, characterised by plumose awns, plus Sarcocaulon; and a second with the 16 species of Monsonia sect. Olopetalum which share the type of mericarp detaching, consistency of columella and the thick mericarp walls. Consequently, some doubts about the monophyly of Monsonia exists. However, the current infrageneric classification of Monsonia is supported after transferring M. longipes and M. speciosa to sect. Olopetalum. Several characters of the mericarp are related to zoochory, which seems to be a derived syndrome in Monsonia, as in the other Geraniaceae.