A new species of meiolaniform turtle, Trapalcochelys sulcata gen. nov. sp. nov. is described, based on material from the late Campanian—early Maastrichtian Allen Formation, Patagonia (Argentina). The postcranial remains recovered are described macroscopically (e.g., external morphology) and microscopically (e.g., histological sections of the shell). Trap alcochelys sulcata gen. nov. sp. nov. shares with other meiolaniforms the presence of sulci strongly curved anteriorly among marginal scales, and dermal bones ornamented with small foramina. This new species differs from the other Late Cretaceous meiolaniform from Patagonia —Patagoniaemys gasparinae— in the general size and in the shape of neural 1. The shell-bone histology is characterized by a diploe structure, in which well developed internal and external compact bone layers frame an area of cancellous bone. Compact bone is mostly composed by interwoven structural fiber bundles. The abundance of structural fibers in the internal cortex and the presence of large pipe-like vascular spaces in the cancellous bone are the most distinctive histological features observed for T. sulcata. All meiolaniform turtle remains of the Upper Cretaceous of South America are exhaustively revised. The known South American record of Upper Cretaceous meiolaniforms is restricted to Argentina and in this revision six localities with outcrops bearing these fossils have been identified. Meiolaniforms are confirmed as a component of the late Campanian—early Maastrichtian South American Allenian tetrapod assemblage.