The reindeer (Rangifer tarandus), also known as caribou in North America, 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. Rangifer herd size varies greatly in different geographic regions. The Taimyr herd of migrating tundra reindeer (R.t. sibiricus) in Russia is the largest wild reindeer herd in the world with numbers varying between 400,000 and 1,000,000. The second largest is the migratory woodland caribou (R.t. caribou) George River herd in Canada, with variations between 28,000 and 385,000.

In traditional festive legend, Santa Claus's reindeer pull a sleigh through the night sky to help Santa Claus deliver gifts to children on Christmas Eve.