The sternum of a very large bony-toothed bird (Pelagornithidae) from the Miocene of Portugal is described. The three-dimensionally preserved specimen is one of the largest sterna of a volant bird known to date, and more complete than the only other pelagornithid sternum reported so far. It is tentatively assigned to Pelagornis miocaenus Lartet, 1857, which is the only bony-toothed bird known from the Miocene of Europe. The specimen shows several unique features, including the presence of a marked cranial projection of the carina sterni, which probably abutted the extremitas sternalis of the furcula, and a large, steep-walled opening in the cranial portion of the facies visceralis. The corpus sterni further has a highly unusual shape in that it is very deep, with a strongly vaulted facies visceralis. It is likely that these characteristics are due to anatomical constraints imposed by the very large size of pelagornithids, which may have reached a wingspan of up to 6 m and probably were incapable of sustained flapping flight.