The almost cosmopolitan distribution of Samolus valerandi is unique in the genus Samolus L. (Samolaceae), which also includes 12–15 taxa with distributions restricted to smaller areas of the Southern Hemisphere. Recent molecular phylogenetic studies based on chloroplast and nuclear DNA sequences showed that the widespread S. valerandi and the North American S. parviflorus are both part of a strongly supported clade, together with the North American S. vagans, S. spathulatus, and S. latifolius. To better understand the origin, distribution, and diversification of S. valerandi and to clarify the relationships within this clade, we performed molecular phylogenetic analyses based on the plastid trnS-G intergenic spacer and the nuclear ribosomal ITS region. We have also sought further support for relationships by examining flower and leaf characters. On the basis of new results, we propose that S. valerandi, S. parviflorus, and S. vagans are considered as part of a widespread “species complex”, with its centre of diversity in North America. No clear vicariance patterns were found regarding the phylogeography of S. valerandi, which thus seems to have dispersed secondarily to various places around the world, possibly as a result of human activities.