Metaxytherium is an extinct genus of dugong that lived from the Oligocene until the end of the Pliocene. Fossil remains have been found in Africa, Europe, North America and South America. Generally marine seagrass specialists, they inhabited the warm and shallow waters of the Paratethys, Mediterranean, Caribbean Sea and Pacific coastline. American species of Metaxytherium are considered to be ancestral to the North Pacific family Hydrodamalinae, which includes the giant Steller's sea cow.

The first remains of Metaxytherium were described in 1822 by Anselme-Gaëtan Demarest as a species of Hippo, H. medius before the genus name Metaxytherium was coined in 1840 by De Christol. Although the type species was initially designated to be M. cuvieri, later publications argued that the two species are synonymous and M. medium thus holds precedence. The grammatical changes of the species name were made to match the rules of the International Code of Zoological Nomenclature (ICZN).

M. serresii was introduced by Gervais in 1847 to encompass the Metaxytherium finds made in Montpellier, France from the Early Pliocene. Although he later described and figured multiple specimens, no type specimen was ever introduced. M. floridanum was named in 1922 by Oliver P. Hay based on a piece of a right maxilla discovered 11 years prior near Mulberry, Florida. The genus Thalattosiren, described by Sickenberg in 1928, was erected from remains previously named M. pettersi, however, further research concluded that it should not only be synonymous with Metaxytherium on a genus level, but is also indistinguishable from M. medium as a species. M. arctodites was named in 1994 by Aranda-Manteca, Domning and Barnes based on three specimens found in California and Baja California, constituting the first valid record of this genus from the north-eastern Pacific. The most recently described species is Metaxytherium albifontanum, described in 2014 by Jorge Vélez-Juarbe and Daryl P. Domning.

The name Metaxytherium means "in-between beast" or "intermediate beast" from the Greek μεταξύ (metaxy) and θηρίον (therion). The name derives from the original interpretation of it being an intermediate form between dugongs and manatees.

There are several genera that are now synonymous with Metaxytherium, including Thalattosiren, Halianassa, Felsinoitherium, Cheirotherium and Hesperosiren. Several species previously considered distinct have likewise been synonymized with already established Metaxytherium species.

There are tentatively referred specimens from the early Miocene Pirabas Formation of Brazil and undetermined remains from the middle Miocene Parana Formation of Argentina.

The exact origins of Metaxytherium are complicated and not entirely understood, with the two basal-most species being known from opposite sides of the Atlantic. Jorge Vélez-Juarbe and Daryl P. Domning propose in their 2014 paper that the closest relatives to the clade comprising dugongs, Metaxytherium and hydrodamalines are native to the Caribbean, suggesting a West-Atlantic Caribbean origin of the genus. This would place Caribosiren or a related genus as a potential ancestor of Metaxytherium. They further suggest that the genus must have originated during the middle Oligocene, due to the two basal most taxa likely having already diverged from one another prior to the Chattian. However, the large distance between these basal species, M. albifontanum and M. krahuletzi, may indicate that their origin is more complex than currently understood.

Research conducted prior to the description of M. albifontanum instead generally suggested a European origin of Metaxytherium, possibly derived from Lentiarenium christolii (previously Halitherium) and eventually forming an anagenetic lineage of European and North African sirenians. While M. krahuletzi and M. medium go through only relatively slight morphological changes, M. serresii represents a strange diversion from the overall trend observed among the European Metaxytherium chronospecies. Generally, the European forms of this genus seemed to grow to progressively larger sizes beginning with M. krahuletzi and culminating in M. subapenninum. M. serresii however is notably smaller than its ancestors and descendants. Originally, Domning and Thomas suggested that this drastic reduction in body size may have been a direct result of the Messinian salinity crisis, creating suboptimal conditions for the European seagrass fields and in turn affecting the sirenian population until the plant life could recover. The drastic increase in tusk size would also support this hypothesis, suggesting that when faced with declining resources, the European Metaxytherium adapted to better exploit what was available, including the rhizomes of seagrasses. However, more recent discoveries from Italy suggest that M. serresii already existed by the time of the Messinian Salinity Crisis, and the dwarfing may instead be a result of the Tortonian Salinity Crisis (7.8 - 7.3 Ma). Although earlier than previously assumed, the circumstances that caused the size decrease have remained similar. The relict species M. subapenninum represents the last stage of evolution of the European Metaxytherium, returning to and even exceeding the body size of the early and middle Miocene species while continuing the increase in tusk size and strengthening rostral reinforcement. Like M. serresii, M. subapenninum was endemic to the Mediterranean following the isolation of the Paratethys that previously supported Metaxytherium. Despite its adaptations to the cooling climate, Metaxytherium subapenninum failed to change its seagrass based diet in the same way the hydrodamalines of the Bering Sea did, eventually dying out towards the end of the Pliocene as temperatures continued to drop.