During an ongoing systematic revision of the Decapoda Grapsoidea (here defined as including the families Gecarcinidae, Grapsidae s. str., Plagusiidae, Sesarmidae, and Varunidae; see Schubart et al., 2000a), it became evident that adult and larval morphology of two Central American genera, Glyptograpsus and Platychirograpsus, differs greatly from that of all other genera within this superfamily. Several important morphological characters are shared by these two genera and represent synapomorphies when compared to the other grapsoids. Both of these genera lack a pleurobranch on the sixth thoracic segment. Adult males of Glyptograpsus and Platychirograpsus are all strikingly heterochelous with the major chela being conspicuously flattened anteriorly and showing a subproximal articulation with the carpus. The distal portion of the male gonopod is uncinate, with a narrowed, elongate terminus. The anteriormost portion of the sternum consists of a narrow, fused sternite terminated in a distinctly flanged tip inserted between the mouth appendages. The male abdomen exhibits immobility in the joints between segments 3 to 5 and very limited mobility in the joint between segments 1 and 2. Zoeae of the two genera show a 1,2 setation pattern on the maxillar endopod. A molecular phylogeny of the Grapsoidea, based on 16S mtDNA and including type genera of the five recognized families, confirms that the species of Glyptograpsus and Platychirograpsus together form a well-defined monophyletic unit that is distinct from all other taxa within the Grapsoidea. We therefore describe a new family, the Glyptograpsidae, to accommodate these two genera of crabs.