The great green macaw (Ara ambiguus), also known as Buffon's macaw or the great military macaw, is a critically endangered Central and South America parrot found in Nicaragua, Honduras, Costa Rica, Panama, Colombia and Ecuador. Two allopatric subspecies are recognized; the nominate subspecies, Ara ambiguus ssp. ambiguus, occurs from Honduras to Colombia, while Ara ambiguus ssp. guayaquilensis appears to be endemic to remnants of dry forests on the southern Pacific coast of Ecuador. The nominate subspecies lives in the canopy of wet tropical forests and in Costa Rica is usually associated with the almendro tree, Dipteryx oleifera.

The great green macaw belongs to the genus Ara, which includes other large parrots, such as the scarlet macaw, the military macaw, and the blue-and-yellow macaw.

This bird was first described and illustrated in 1801 by the French naturalist François Le Vaillant for his Histoire Naturelle Des Perroquets under the name "le grand Ara militaire", using a skin deposited at the Muséum national d'histoire naturelle in Paris. Le Vaillant states that it is not certain if the bird is truly a distinct species of parrot, or, as he thinks more likely, it is specific varietal race of the military macaw, but nonetheless, he must mention that its existence merits notice. The bird was subsequently named Psittacus ambiguus by the Thuringian Johann Matthäus Bechstein in the first tome of the fourth volume, published in 1811, of the series Johann Latham's Allgemeine Uebersicht der Vögel, the greatly expanded German translation of the Englishman John Latham's A General Synopsis of Birds. Bechstein mentions le Vaillant's reluctance to consider it as an independent species, but explains that having examined a living bird, he considers it a valid species, mentioning the size difference and enumerating numerous other characteristics he deems distinctive.

After almost 200 years, the binomial name was changed from Ara ambigua to Ara ambiguus in 2004, as it was decided that the word ara was in fact male, despite ending in an -a (see epicene).

There are two subspecies which are geographically isolated at present: Ara ambiguus ssp. ambiguus, which has the largest distribution range (Central and northern South America), and Ara ambiguus ssp. guayaquilensis, which only occurs in Ecuador. The Ecuadorian subspecies is sometimes referred to as Chapman's macaw or Chapman's green macaw. American naturalist Frank M. Chapman shot the type specimen of his proposed new taxon in 1922 on a hill in the Cordillera de Chongon, twenty miles northwest of Guayaquil, Ecuador, and first described the taxon in 1925 in a report on the newly collected bird skins he had brought back to the US from Ecuador.

Due to the morphological variability of ssp. guayaquilensis, with a number of specimens of this taxon being identifiable as the military macaw, in 1996 Berg and Horstman, themselves referencing Fjeldså et al.., mentioned it might best be synonymised with A. militaris, or suggested there might be gene-flow between all three populations. A 2015 study comparing the mitochondrial DNA of different populations of the military macaw and this species found that while these two species are clearly differentiated, as well as different populations of the military macaw in Mexico, no genetic difference between ssp. guayaquilensis and the nominate taxon was found (at least regarding the mitochondria). This indicates that the division of this species into two subspecies is likely not taxonomically valid. It is also possible that the Ecuadorian populations do not all belong to ssp. guayaquilensis.

Great green macaws are the largest parrots in their natural range, the second heaviest macaw species (although they are relatively shorter tailed than other large macaws such as the red-and-green macaw and are thus somewhat shorter), and the third heaviest parrot species in the world. This species averages 85–90 cm (33.5–35.5 in) in length and 1.3 kg (2.9 lb) in weight. They are mainly green and have a reddish forehead and pale blue lower back, rump and upper tail feathers. The tail is brownish-red tipped with very pale blue. The bare facial skin is patterned with lines of small dark feathers, which are reddish in older and female parrots. Juveniles have grey-coloured eyes instead of black, are duller in colour and have shorter tails which are tipped in yellow.

The main morphological distinction with the subspecies guayaquilensis is that this bird has a smaller, narrower bill.