Megaloceros (from Greek: μεγαλος megalos + κερας keras, literally "Great Horn"; see also Lister (1987)) is an extinct genus of deer whose members lived throughout Eurasia from the Pleistocene to the early Holocene. The type and only undisputed member of the genus, Megaloceros giganteus, vernacularly known as the "Irish elk" or "giant deer", is also the best known. Fallow deer are thought to be their closest living relatives. Megaloceros has been suggested to be closely related to other genera of "giant deer", like the East Asian genus Sinomegaceros (whose species are sometimes included in Megaloceros), and the European Praemegaceros.

Megaloceros giganteus was originally described in 1799 as Alce gigantea by Johann Friedrich Blumenbach based on specimens found in Ireland. With Alce being a variant of the genus Alces used for elk/moose.

In 1827 Joshua Brookes, in a listing of his zoological collection, named the Megaloceros (spelled Megalocerus in the earlier editions) in the following passage:

Amongst other Fossil Bones, there [are] ... two uncommonly fine Crania of the Megalocerus antiquorum (Mihi). (Irish), with unusually fine horns, (in part restored)

— Joshua Brookes, Brookesian Museum. The Museum of Joshua Brookes, Esq. Anatomical and Zoological Preparations, p 20.

The etymology being from Greek: μεγαλος megalos "great" + κερας keras "horn, antler". The type and only species named in the description being Megaloceros antiquorum, based on Irish remains now considered to belong to M. giganteus, making the former a junior synonym. The original description was considered by Adrian Lister in 1987 to be inadequate for a taxonomic definition. In 1828 Brookes published an expanded list in the form of a catalogue for an upcoming auction, which included the Latin phrase "Cornibus deciduis palmatis" ("palmate deciduous horns") as a description of the remains. The 1828 publication was approved by International Commission on Zoological Nomenclature (ICZN) in 1977 as an available publication for the basis of zoological nomenclature. Adrian Lister in 1987 judged that "the phase "Cornibus deciduis palmatis" constitutes a definition sufficient under the [International Code of Zoological Nomenclature] (article 12) to validate Megalocerus." The original spelling of Megalocerus was never used after its original publication.

In 1844 Richard Owen named another synonym of the Irish elk, including it within the newly named subgenus Megaceros, Cervus (Megaceros) hibernicus. This has been suggested to be derived from another junior synonym of the Irish elk described by J. Hart in 1825, Cervus megaceros. Despite being a junior synonym, Megaloceros remained in obscurity and Megaceros became the common genus name for the taxon. The combination "Megaceros giganteus" was in use by 1871. George Gaylord Simpson in 1945 revived the original Megaloceros name, which became progressively more widely used, until a taxonomic decision in 1989 by the ICZN confirmed the priority of Megaloceros over Megaceros, and Megaloceros to be the correct spelling.

Other than the type species Megaloceros giganteus, the composition of the genus is contested. While considered to be part of the genus Megaloceros by many authors, M. savini and related taxa (novocarthaginiensis and matritensis) are split into the separate genus Praedama by some scholars.