犎牛（學名Bison bison）又名美洲水牛或美洲野牛，是牛亞科哺乳動物，也是北美洲體型最大的哺乳動物和世界上最大野牛之一。儘管體型龐大，仍可維持60公里的奔跑速度。主要群體由雌牛和幼牛組成；雄性會另外組成單身漢群，只有交配時才會聚集在一起。平時以嫩莖晘草為食，無領域性。美洲森林野牛一般被認為是其亞種，但分類地位仍有爭議。最近根據最新DNA研究顯示，牠和歐洲野牛親源關係比所知還要接近。

體型龐大的美洲森林野牛是北美洲最大的亞種，牠的體型大過亞洲野牛和野生水牛，後兩者主要出現在印度。野牛大量集體居住在美國和加拿大的大平原，由加拿大遠北的大奴湖至南面的墨西哥，再由奧勒崗州東部至大西洋一帶。其兩個亞種分別是美洲草原野牛（Bison bison bison），體型較小和有較圓滑的背部隆肉，和美洲森林野牛，體型較大和較高和方形的背部隆肉。

The American bison (Bison bison), also commonly known as the American buffalo, is a North American species of bison that once roamed the grasslands of North America in massive herds. They became nearly extinct by a combination of commercial hunting and slaughter in the 19th century and introduction of bovine diseases from domestic cattle, and have made a recent resurgence largely restricted to a few national parks and reserves. Their historical range roughly comprised a triangle between the Great Bear Lake in Canada's far northwest, south to the Mexican states of Durango and Nuevo León, and east to the Atlantic Seaboard of the United States (nearly to the Atlantic tidewater in some areas) from New York to Georgia and per some sources down to Florida. Bison were seen in North Carolina near Buffalo Ford on the Catawba River as late as 1750.

美國總統奧巴馬20160509日正式簽署法案，讓北美野牛成為美國的「國家哺乳動物」，確認北美野牛的地位，和美國國鳥白頭海鵰（bald eagle）一樣，成為美國遺產的一環。

歐巴馬簽署確認後，北美野牛瞬間躍升為「國獸」，其實在此之前，北美野牛的形象早已深植美國社會，例如美國內政部的印章圖案從1912年起，就刻有北美野牛的圖案，此外5分美元背面也刻有北美野牛的圖案。美國國會在上月通過這項法案時，法案起草之一的眾議員就指出，「沒有其他本土物種比這種高貴的動物（北美野牛）更能講述美國故事。」