Alismatidae is a wetland or aquatic herb lineage of monocots with a cosmopolitan distribution. Although considerable progress in systematics and biogeography has been made in the past several decades, geographical origin of this group remains unresolved. In this study, we used statistical dispersal–vicariance analysis implemented in program RASP to investigate the biogeography of Alismatidae. Six areas of endemism were used to describe the distribution: North America, South America, Eurasia, Africa, Southeast Asia and Australia. 18,000 trees retained from Bayesian inference of rbcL served as a framework to reconstruct the ancestral areas. The results suggested that the most recent common ancestor of Alismatidae most probably occurred in Eurasia, followed by a split into two major clades. The clade comprising Hydrocharitaceae, Butomaceae and Alismataceae mainly diversified in Eurasia and Africa. The other clade comprising the remaining families dispersed to southern hemisphere. Australia played an important role in diversification of this clade. Several families were suggested to have occurred in Australia, such as Ruppiaceae, Cymodoceaceae, Posidoniaceae and Zosteraceae.