Stegotherium is an extinct genus of long-nosed armadillo, belonging to the Dasypodidae family alongside the nine-banded armadillo. It is currently the only genus recognized as a member of the tribe Stegotheriini. It lived during the Early Miocene of Patagonia and was found in Colhuehuapian rocks from the Sarmiento Formation, Santacrucian rocks from the Santa Cruz Formation, and potentially also in Colloncuran rocks from the Middle Miocene Collón Curá Formation. Its strange, almost toothless and elongated skull indicates a specialization for myrmecophagy, the eating of ants, unique among the order Cingulata, which includes pampatheres, glyptodonts and all the extant species of armadillos.

Stegotherium tessellatum was described originally in 1887 by Florentino Ameghino based on the remains of a carapace collected by his brother Carlos in the Santa Cruz Province of Argentina. The same paper also described another genus and species of armadillo, Scaetops simplex, known from a fragmentary mandible. In 1894, Stegotherium, at that time only known from osteoderms, was temporarily considered by Lydekker as a synonym of Peltephilus. This status was contested and proven wrong a year later by Ameghino.^{[citation needed]}

In 1902, after a skull of Scaetops simplex was found in association with Stegotherium tessellatum osteoderms, Ameghino considered the two species synonymous, and proposed a new species Stegotherium variegatum based on osteoderms found in Chubut Province. In 1904, after the discovery of additional remains of S. variegatum, William Berryman Scott re-evaluated Scaetops simplex as a species of Stegotherium different from S. tessellatum.

In 2008, two important studies on the genus were published. The first, led by Fernicola and Vizcaíno, reviewed the material and species assigned to the genus. They proposed two new species, S. caroloameghinoi, with MACN-A 10443a, an osteoderm from the dorsal carapace, as holotype, and S. pascuali using MACN A-12680d, an osteoderm from the dorsal carapace, as holotype. This review also kept, not without some doubt, S. simplex as a valid taxon. The second study from 2008, led by González Ruiz and Scillato-Yané, proposed ‘’Stegotherium tauberi’’ as a species, based on YPM PU 15565, a fairly complete specimen including a fragmentary dorsal carapace, a complete skull, several vertebra and a right foot, previously assigned to S. tessellatum.

In 2009, another species was named by González Ruiz and Scillato-Yané, S. notohippidensis, with the holotype being MLP 84-III-5-10, a collection of 130 osteoderms from Argentina.

Stegotherium was an unusual armadillo, whose most striking feature was the elongated skull, often compared to the skull of an anteater. The posterior area of the jaws, the only one to bear teeth, was compressed compared to Dasypus, while the nasal area and the anterior parts of both jaws, completely toothless, were long and slender. The teeth were cylindricals and greatly reduced, both in number and in size, and were all contained in the posterior area of the lower and upper jaws. While S. tauberi had six teeth in its lower mandible, the dubious S. simplex only had two.

The body of Stegotherium was roughly the size of the modern species of Dasypus, and its carapace was composed of at least 23 mobile bands of osteoderms. The osteoderms of Stegotherium, 3 to 7.5 mm thick and 20 mm long, were characterized by the presence of a number of piliferous foramina around their posterior and lateral margins, a granular appearance, and a compact bone structure.

The genus Stegotherium is unambiguously known from six species, S. tessellatum, S. variegatum, S. caroloameghinoi, S. pascuali, S. tauberi and S. notohippidensis. A seventh species, S. simplex, is generally considered too fragmentary, but has generally been considered valid with reservations by most recent scholars. As osteoderms are the most abundant fossils of Stegotherium known, they are commonly used as the main determinate of which species a given fossil belongs too.^{[citation needed]}