Gargantuavis (meaning 'gargantuan bird') is an extinct genus of large, primitive bird containing the single species Gargantuavis philoinos. It is the only member of the monotypic family Gargantuaviidae. Its fossils were discovered in several formations dating to 73.5 and 71.5 million years ago in what is now northern Spain, Southern France, and Romania. Gargantuavis is the largest known bird of the Mesozoic, a size ranging between the cassowary and the ostrich, and a mass of 141 kg (311 lb) like modern ostriches, exemplifying the extinction of non-avian dinosaurs was not a necessary condition for the emergence of giant terrestrial birds. It was once thought to be closely related to modern birds, but the 2019 discovery of a pelvis identified as cf. Elopteryx nopcsai from what was Hațeg Island (present-day Romania) shows several primitive features.

Its femur shows that it was a graviportal form rather than cursorial, not adapted for running. Due to fragmentary remains, many aspects of its biology and ecology are unknown, such as its diet. It coexisted with large predators like abelisaurid theropods, herbivores such as ankylosaurians and titanosaurian sauropods, as well as pterosaurs, crocodylomorphs, turtles, fish, and various archaic birds.

The first Gargantuavis fossil was found in 1995 in Var, southeastern France. This first specimen, a fragmentary set of pelvic vertebrae (synsacrum), was uncovered near the village of Fox-Amphoux in a paleontological excavation and described by French paleontologists Eric Buffetaut and Jean Le Loeuff, who noted the synsacrum's similarity to that of modern birds. Several other specimens were later found further west, near the villages of Villespassans, Cruzy, and Campagne-sur-Aude, providing enough fossil material to describe and name the species in 1998. The genus name refers to Gargantua, the giant and protagonist of the 16th-century French novel The Life of Gargantua and of Pantagruel by Francois Rabelais, and the Latin avis. The species name philoinos, meaning "one who likes wine", was chosen because several of the first Gargantuavis bones were found in and around vineyards and wineries.

Gargantuavis specimens are known from six localities in Europe:

Though Gargantuavis is only known from a few isolated fossil bones, some information about its life appearance and ecology have been inferred by studying their details. Gargantuavis is known from several specimens representing a few limited parts of the skeleton: synsacra (the fused vertebrae above the hip), ilia (hip bones), at least one cervical vertebra, and two femora (upper leg bone), which was referred to the species based on the fact that it seems to fit well with the hip. No cranial remains have been found, so the shape of the head is unknown. However, the only known cervical vertebra suggests that Gargantuavis had a rather long and slender neck, which seems to preclude the presence of a massive skull.

Other than its large size, the most unusual feature of Gargantuavis was its pelvis. It was originally reported to be extremely wide, like that of a moa, though a better preserved specimen described in 2015 showed that this interpretation was due to crushing in the original. The hips of Gargantuavis, while still broad, were narrower and more bird-like than originally thought. In addition to their unusual width, which prevented the two ilia from meeting at the front of the pelvis, the hip socket was set close to the front rather than to the middle of the pelvis. The rather broad pelvis shows that Gargantuavis was not a fast runner.

During the Late Cretaceous, Europe was an archipelago. Southern France and north-western Spain where its fossils are found was part of the large Ibero-Armorican island in the prehistoric Tethys Sea. The rock formations that have yielded Gargantuavis fossils have also produced abundant remains of fish, turtles, crocodylomorphs, pterosaurs, various titanosaurian sauropods (including Ampelosaurus and Lirainosaurus), ankylosaurians, ornithopods, and theropods, including other early avialans, like enantiornithes. The association of abundant fossils of the ornithopod Rhabdodon, and the lack of any hadrosaurid fossils, have been used as index fossils to roughly date these formations to the late Campanian-early Maastrichtian interval, an age confirmed later by magnetostratigraphic evidence in two localities. The type locality of Gargantuavis, the Bellevue site in the Marnes Rouges Inferieures Formation, is 71.5 million years old (earliest Maastrichtian). The Spanish site of Laño is slightly older with an age of 72 to 73.5 Ma (latest Campanian).

Since no skull remains have been found, the diet of the animal is uncertain. Contrary to the giant terrestrial Cenozoic birds that lived in ecosystems without predators (or including only small carnivores), Gargantuavis cohabited with abelisaurid and dromaeosaurid theropods, so the place of this giant terrestrial bird in the Late Cretaceous ecosystems of the Ibero-Armorican island is unclear. Gargantuavis seems to have been an uncommon part of the fauna in its region. Despite numerous digs at sites where its bones have been found since its discovery, most have yielded only single specimens. Although its fossils are rare, the presence of Gargantuavis from southeastern France to north-western Spain shows that this bird had a wide distribution in the Ibero-Armorican island. It is possible that Gargantuavis lived mainly in an environment that was not compatible with fossilization, such as areas far from the rivers and floodplains, which represent most of the fossiliferous deposits in the late Campanian-early Maastrichtian of France and Spain.