Ammonite, any one of a group of shelled sea animals that became extinct millions of years ago. Ammonites were mollusks whose shells ranged in size from about one-half inch (13 mm) to more than six feet (1.8 m) in diameter. They are known today only from the fossilized remains of their shells. Ammonite shells resemble the shells of the pearly nautilus, a living sea creature, in that they consist of a succession of chambers, with the animal occupying the outermost chamber. Ammonites flourished during the Triassic and Jurassic periods, and became extinct about 65 million years ago, at the end of the Cretaceous period.

The name “ammonite” comes from the fact that many of the shells resemble the horns of the ancient Egyptian god Ammon, who sometimes was represented as a man with a ram's head.

Ammonites make up the suborder Ammonoidea of the class Cephalopoda, phylum Mollusca.