灰熊（学名：Ursus arctos horribilis）为棕熊亚种，分布于北美洲西部，是北美最常见的棕熊。灰熊的祖先被认为是10万年前从西伯利亚跨过白令海峡来到阿拉斯加的乌苏里棕熊。

灰熊通常独居生活，但是在沿海地区，灰熊在鲑鱼产卵的季节会聚集在溪流、河流、湖泊和池塘周围。雌性灰熊每隔一年生产1至4头幼崽，通常为2头，每头幼崽重量仅为500克。母熊严密保护其幼崽，在认为自己或幼崽受到威胁时，便会进攻。

The grizzly bear (Ursus arctos ssp.) is any North American morphological form or subspecies of brown bear, including the mainland grizzly (Ursus arctos horribilis), Kodiak bear (U. a. middendorffi), peninsular grizzly (U. a. gyas), and the recently extinct California grizzly (U. a. californicus†) and Mexican grizzly bear (U. a. nelsoni†). Scientists do not use the name grizzly bear but call it the North American brown bear. (See brown bear for a discussion of brown bears outside of North America). It should not be confused with the black grizzly or Ussuri brown bear (U. a. lasiotus) which is another giant brown bear inhabiting Russia, Northern China, and Korea.