A molecular phylogeny is used to analyze the diversification history of Adesmia ser. psoraleoides, and its implications for understanding the historical assembly of the grasslands in the highlands of southern Brazil. All species of A. ser. psoraleoides were sampled, including multiple accessions for each species, plus representative species of the rest of Adesmia covering its geographic distribution. Phylogenetic analyses were based on nuclear and plastid DNA sequences, and a plastid matK phylogeny was dated. A. ser. psoraleoides is a well-supported monophyletic group, nested within the series bicolores, muricatae, subnudae, longisetae and candidae. The stem of A. ser. psoraleoides is c. 11 Mya, but most extant species diversified c. 3–5 Mya, suggesting that the flora of the subtropical grasslands of southern Brazil was assembled recently, co-incident with the expansion of other tropical grassland systems globally.