| Named By: | Hebert in 1855 |
| Time Period: | Paleocene - Eocene, 56-45 Ma |
| Location: | Belgium. China. England. France. Germany. USA |
| Size: | Largest individuals easily up to 2 meters tall |
| Diet: | Uncertain, but probably herbivorous, refer to main text for clarification |
| Fossil(s): | Numerous individuals of varying levels of completeness, but so many fossils have now been discovered that the form of Gastornis is now known without doubt |
| Classification: | | Chordata | Aves | Anseriformes | Gastornithidae | |
| Also known as: | | Barornis | Diatryma | Gastornis eduardsii | Gastornis minor | Omorhamphus | Zhongyuanus | |
Gastornis is an extinct genus of large flightless birds that lived during the late Paleocene and Eocene epochs of the Cenozoic era. The genus is currently thought to contain three or four distinct species, known from incomplete fossil remains, found in western-central Europe (England, Belgium, France and Germany). More complete specimens are known from a fourth, North American species, which had previously been classified in the distinct genus Diatryma. Many scientists now consider Diatryma to be so similar to the other species of Gastornis that it should also be included in that genus. A fifth species, also previously classified in its own genus, is known from China.
Gastornis species were very large birds, and have traditionally been considered to be predators of small mammals. However, several lines of evidence, including the lack of hooked claws in known Gastornis footprints and studies of their beak structure have caused scientists to reinterpret these birds as herbivores that probably fed on tough plant material and seeds.