Tytthostonyx is a genus of prehistoric seabird. Found in the much-debated Hornerstown Formation which straddles the Cretaceous–Paleogene boundary 66 million years ago, this animal was apparently closely related to the ancestor of some modern birds, such as Procellariiformes and/or "Pelecaniformes". A single species is placed herein, Tytthostonyx glauconiticus. It has been placed into a family of its own, Tytthostonygidae.