犎牛（学名Bison bison）又名美洲水牛或美洲野牛，是牛亚科哺乳动物，也是北美洲体型最大的哺乳动物和世界上最大野牛之一。尽管体型庞大，仍可维持60公里的奔跑速度。主要群体由雌牛和幼牛组成；雄性会另外组成单身汉群，只有交配时才会聚集在一起。平时以嫩茎晘草为食，无领域性。美洲森林野牛一般被认为是其亚种，但分类地位仍有争议。最近根据最新DNA研究显示，牠和欧洲野牛亲源关系比所知还要接近。

体型庞大的美洲森林野牛是北美洲最大的亚种，牠的体型大过亚洲野牛和野生水牛，后两者主要出现在印度。野牛大量集体居住在美国和加拿大的大平原，由加拿大远北的大奴湖至南面的墨西哥，再由奥勒岗州东部至大西洋一带。其两个亚种分别是美洲草原野牛（Bison bison bison），体型较小和有较圆滑的背部隆肉，和美洲森林野牛，体型较大和较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.