The reindeer or caribou (Rangifer tarandus) is a species of deer with circumpolar distribution, native to Arctic, subarctic, tundra, boreal, and mountainous regions of Northern Europe, Siberia, and North America. This includes both sedentary and migratory populations. It is the only representative of the genus Rangifer. Herd size varies greatly in different geographic regions. More recent studies suggest the splitting of reindeer and caribou into six distinct species over their range.

Reindeer occur in both migratory and sedentary populations, and their herd sizes vary greatly in different regions. The tundra subspecies are adapted for extreme cold, and some are adapted for long-distance migration.

Reindeer vary greatly in size and color from the smallest, the Svalbard reindeer (R. (t.) platyrhynchus), to the largest, Osborn's caribou (R. t. osborni). Although reindeer are quite numerous, some species and subspecies are in decline and considered Vulnerable. They are unique among deer (Cervidae) in that females may have antlers, although the prevalence of antlered females varies by species and subspecies.

Reindeer are the only successfully semi-domesticated deer on a large scale in the world, and both wild and domestic reindeer have been an important source of food, clothing, and shelter for Arctic people throughout history and are still herded and hunted today. Wild reindeer "may well be the species of single greatest importance in the entire anthropological literature on hunting." In some traditional Christmas legends, Santa Claus's reindeer pull a sleigh through the night sky to help Santa Claus deliver gifts to good children on Christmas Eve.