Ossancora, new genus, is diagnosed within Doradidae by having the unique combination of posterior coracoid process approximately as long as posterior cleithral process in adults, posterior cranial fontanel occluded, long maxillary barbel with smooth elongate fimbriae, and teeth present on dentary and premaxilla. A similarly long posterior coracoid process is found in only one other doradid species, Trachydoras paraguayensis, which is distinguished, as are its congeners, from Ossancora by having a small posterior cranial fontanel retained in adults, a relatively short maxillary barbel with stout fimbriae, and edentulous jaws. Three nominal species are recognized as valid and redescribed in Ossancora: Doras punctatus Kner 1853, in both the Amazonas and Paraná-Paraguay basins, Oxydoras eigenmanni Boulenger 1895, in the Paraguay basin, and Doras fimbriatus Kner 1855 in the Amazonas basin. The younger name Doras fimbriatus Kner 1855 is considered valid as a nomen protectum according to Article 23.9 of the ICZN, and its older synonym, D. loricatus Kner 1853 qualifies as a nomen oblitum. A fourth new species, Ossancora asterophysa, is described and diagnosed in part by a gas bladder morphology that is distinctive among congeners. Three species, O. eigenmanni, O. fimbriata and O. asterophysa, form a monophyletic group based on three shared characteristics: smooth elongate fimbriae inserted in more than one row, one dorsally and one ventrally, along anterior margin of maxillary barbel (uniquely derived among Doradidae), elongate fimbriae on mental barbels, and anterior nuchal plate reduced, not sutured to epioccipital (latter two features uniquely derived among Ossancora). Ossancora is typical of lowland floodplain lakes and river channels, and its occurrence in the both the Amazonas and Paraná-Paraguay supports a historical link between these two basins. A key to species and detailed anatomical descriptions are provided. Type specimens are discussed and lectotypes are newly designated for Oxydoras eigenmanni Boulenger 1895, D. fimbriatus Kner 1855, and Doras (Corydoras) punctatus Kner 1853, respectively.