Freshwater crab sister group relationships with marine eubrachyuran families were investigated. A morphology-based cladistic analysis was conducted on representatives of the freshwater crab families Deckeniidae, Gecarcinucidae, Parathelphusidae, Potamidae, Potamonautidae, Pseudothelphusidae, and Trichodactylidae using a disparate assemblage of marine heterotreme and thoracotreme brachyurans as possible sister groups. The monophyly of the freshwater crabs sensu lato is falsified. The family Trichodactylidae and the marine portunid subfamily Carcininae form basal groups within the superfamily Portunoidea. The monophyly of the Pseudothelphusidae and the Paleotropical freshwater crab families is supported, and this clade is the sister group of the Thoracotremata (Gecarcinidae, Grapsidae s.l., and Ocypodoidea). The origin, groundplan, and diversification of freshwater crabs are discussed in the context of previously published scenarios of their evolution.