The grizzly bear (Ursus arctos horribilis), also known as the North American brown bear or simply grizzly, is a population or subspecies of the brown bear inhabiting North America.
In addition to the mainland grizzly (Ursus arctos horribilis), other morphological forms of brown bear in North America are sometimes identified as grizzly bears. These include three living populations—the Kodiak bear (U. a. middendorffi), the Kamchatka bear (U. a. beringianus), and the peninsular grizzly (U. a. gyas)—as well as the extinct California grizzly (U. a. californicus†), Mexican grizzly (formerly U. a. nelsoni†), and Ungava-Labrador grizzly (formerly U. a. ungavaesis†). On average, grizzly bears near the coast tend to be larger while inland grizzlies tend to be smaller.
The Ussuri brown bear (U. a. lasiotus), inhabiting Russia, Northern China, Japan, and Korea, is sometimes referred to as the "black grizzly", although it is no more closely related to North American brown bears than other subspecies of the brown bear around the world.
Meriwether Lewis and William Clark first described it as grisley, which could be interpreted as either "grizzly" (i.e., "grizzled"—that is, with grey-tipped hair) or "grisly" ("fear-inspiring", now usually "gruesome"). The modern spelling supposes the former meaning; even so, naturalist George Ord formally classified it in 1815 as U. horribilis for its character.
Classification has been revised along genetic lines. There are two morphological forms of Ursus arctos: the grizzly and the coastal brown bears, but these morphological forms do not have distinct mtDNA lineages.
Brown bears originated in Eurasia and traveled to North America approximately 50,000 years ago, spreading into the contiguous United States about 13,000 years ago. The genome of the grizzly bear was sequenced in 2018 and found to be 2,328.64Mb (mega-basepairs) in length, and contain 30,387 genes.
In the 19th century, the grizzly was classified as 86 distinct species. However, by 1928 only seven grizzly species remained, and by 1953, only one species remained globally.