Giant anteaters belong to the order Edentata (also known as Xenartha). Edentata is Latin for "without teeth" ("e-" means without and "dent" means teeth). Giant anteaters are part of the family Myrmecophagidae. (Myrmeco means "ant" and phagus means "eating" in Greek.) Giant anteaters' full species name is Myrmecophaga tridactyla. Roughly translated from Greek/Latin, this means "ant-eating with three fingers." (Tridactyla comes from "tri" and "dactyls," or three fingers). This is curious naming, since giant anteaters actually have four toes on each paw, with an additional vestigial fifth. But it probably came about because only three of the four front toes are prominently clawed; the other isn't easily seen.

Edentates diverged from insectivores during the Cretaceous (135 million years ago) in South America and were greatly diversified already by the end of the end of the Age of Dinosaurs (beginning of Age of Mammals) some 65 million years ago. The Myrmecophagidae family is known only back to the Early Miocene (25 million years ago) in South America, though the fossil record is poor so the group may go back a good deal further.

Other anteater species in the Myrmecophagidae family are the semi-arboreal tamandua and the completely arboreal silky anteater. Both are much smaller, and have smaller snouts, though they are similar in general shape. The anteaters are all also related to two-toed and three-toed sloths, which are part of the edentate family Bradypodidae.

A number of unrelated mammals are often misnamed or mistaken for anteaters; they include the aardvark, echidna, armadillo, pangolin, and numbat ("banded" anteater), which is a marsupial. In fact, evolution has produced several completely distinct lines of anteater-like species across the world that look similar and have the same general body functions, but there is only one true giant anteater.

Giant anteaters appear to have pretty much stayed put throughout evolutionary history, not ranging much farther than northern Central America, most likely because of cool weather and changing forests. One fossil of the giant anteater, however, was identified from an early Pleistocene (600,000 years ago) site in northwestern Sonora, Mexico, more than 3,000 kilometers north of the present range of the species.