Klaxon is a trademark for an electromechanical horn or alerting device. The sound is produced by a spring-steel diaphragm with a rivet in the centre which is repeatedly struck by the teeth of a rotating cog-wheel. In the first klaxons the wheel was driven either by hand or by an electric motor. (The electric version has been credited to inventor Miller Reese Hutchison, an associate of Thomas Edison). The diaphragm is attached to a horn which amplifies the sound.

The Lowell-McConnell Manufacturing Co. of Newark, New Jersey bought the rights to the device in 1908. F. W. Lowell, the founder, coined the name klaxon from the Greek word klazō meaning 'I shriek'.

Klaxons were first fitted to automobiles and bicycles in 1908. Electric klaxons were the first electrical devices to be fitted to private automobiles. They were originally powered by 6-volt dry cells, and from 1911 by rechargeable batteries. Later mains-powered versions were used as military evacuation alarms and factory sirens. The klaxon is also famous for its use as a submarine dive alarm.

The English company Klaxon Signals Ltd. has been based in Oldham, in England for the last 80 years. The French Klaxon company was acquired by the Italian Fiamm Group in the 1990s.

The Klaxon: March of the Automobiles was composed by H. Fillmore in 1929 for the Cincinnati Automobile Show, and was originally performed on twelve automobile horns.