美国短吻鳄（学名：Alligator mississippiensis），又称密西西比河鳄，是西半球大型的鳄鱼物种。

雄性鳄长达4米以上，雌的不到3米。幼鳄为黑色，身上点缀着无规律的黄色横向带纹，到了成年时就会消失，因为黄色斑纹会逐渐被黑色素及藻类屏蔽。

美国短吻鳄有圆而宽的口鼻部，上颌每侧有齿17-22枚，下颌每侧17-22枚；躯干部背面的角质鳞共有18横列，其中8列较大；趾间有不完全的蹼。其体色与美洲鳄（Crocodylus acutus）不同，后者的体色为棕色，而且口鼻部较尖细，与原产自中国的扬子鳄有亲缘关系。

The American alligator (Alligator mississippiensis), sometimes referred to colloquially as a gator or common alligator, is a large crocodilian reptile endemic to the southeastern United States. It is one of two living species in the genus Alligator within the family Alligatoridae; it is larger than the other extant alligator species, the Chinese alligator. Adult male American alligators measure up to 3.4 to 4.6 m (11 to 15 ft) in length, and can weigh up to 453 kg (999 lb). Females are smaller, measuring around 3 m (9.8 ft). The American alligator inhabits freshwater wetlands, such as marshes and cypress swamps from Texas to North Carolina. It is distinguished from the sympatric American crocodile by its broader snout, with overlapping jaws and darker coloration, and is less tolerant of saltwater but more tolerant of cooler climates than the American crocodile, which is found only in tropical climates.