The Irish Elk or Giant Irish Deer (Megaloceros giganteus), was a species of Megaloceros and one of the largest deer that ever lived. Its range extended across Eurasia, from Ireland to east of Lake Baikal, during the Late Pleistocene. The latest known remains of the species have been carbon dated to about 7,700 years ago. Its common name, Irish Elk, is misleading as the species was never limited to only Ireland. Although large numbers of skeleton have been found in Irish bogs, the animal was not exclusively Irish, and neither was it closely related to either of the living species currently called elk; for this reason, the name "Giant Deer" is preferred in more recent publications. Megaloceros giganteus first appeared about 400,000 years ago. It possibly evolved from M. antecedens. The earlier taxon — sometimes considered a paleosubspecies M. giganteus antecedens — is similar but had more compact antlers.
The Irish Elk stood about 2.1 metres (6.9 ft) tall at the shoulders, and it had the largest antlers of any known cervid (a maximum of 3.65 m (12.0 ft) from tip to tip and weighing up to 40 kilograms (88 lb)). In body size, the Irish Elk matched the extant Moose subspecies of Alaska (Alces alces gigas) as the largest known deer. A significant collection of M. giganteus skeletons can be found at the Natural History Museum in Dublin.