A postilion or postillion is a person who rides a harnessed horse that is pulling a horse-drawn vehicle such as a coach, rather than driving from behind as a coachman does. This method is used for pulling wheeled vehicles that do not have a driver's seat, such as many ceremonial state coaches and artillery limbers and caissons. Postilion riders are generally arranged one rider for each pair of horses, riding the left horse.

Originally the English name for a guide or forerunner for the post (mail) or a messenger, it became transferred to the actual mail carrier or messenger and also to a person who rides a (hired) post horse. The same persons made themselves available as a less expensive alternative to hiring a coachman, particularly for light, fast vehicles.

A carriage or coach that was arranged without a driver's seat and intended for guidance by postilions, had à la Daumont appended, such as "coach à la Daumont". Daumont is a corruption of the French d'Aumont from the 8th Duke of Aumont who preferred this manner of travel.

Postilions ride the left or nearside mount because horses are mounted from the left. With a double team there could be two postilions, one for each pair, or, especially in France, one postilion would ride on the left wheel (rear) horse in order to control all four horses.

The postilion wears a full-dress livery with a short jacket reaching to the waist only and decorated with gold lace and gilt buttons. A white shirt and stock tie, white leather breeches, white gloves, decorated cap, boots with brown tops, and an iron leg-guard on the [right] leg to protect it from the battering of the carriage pole.

— Thomas Ryder in Fashion on Wheels

This style of travel was known as "posting". The postilions and their horses (known as "post-horses") would be hired from a "postmaster" at a "post house". The carriage would travel from one post house to the next (a journey known as a "stage"), where the postilions and/or spent (exhausted) horses could be replaced if necessary. In practice unless a return hire was anticipated a postilion of a spent team frequently was also responsible for returning them to the originating post house.

Posting was once common both in England and in continental Europe. In addition to a carriage's obvious advantages (a degree of safety and shelter for the inside passengers and accessibility to non-riders) on long trips it tended to be the most rapid form of passenger travel. Individually mounted riders are subject to their personal endurance limits, while posting could continue indefinitely with brief stops for fresh horses and crew. In England, posting declined once railways became an alternative method of transport, but it remained popular in France and other countries.