The A40 is a major trunk road connecting London to Fishguard, Wales and officially called The London to Fishguard Trunk Road (A40) in all legal documents and Acts. It is 256 miles (412 km) long.

It is one of the few remaining of the "old" trunk routes not to have been superseded by a direct motorway link, though some parts, such as the southern section from London to Oxford are now better served by the M40. Part of the A40 forms a section of the unsigned Euroroute E30, which the Welsh Assembly Government refers to as "one of the lowest standard sections of the Trans European Road Network in the United Kingdom"

The original (1923) route of the A40 was the City of London to Fishguard. The road still begins and ends in the same places, but a number of changes have been made to its route.

The first change dates from 1935, between Ross-on-Wye and Abergavenny. The original route of the A40 was via Skenfrith; this road was renumbered the B4521. The A40 was rerouted via Raglan; between Ross and Raglan it replaced part of the A48, between Raglan and Llanvihangel-nigh-Usk it replaced the B4234, and between Llanvihangel and Abergavenny it replaced part of the A471.