Trunk roads in Wales were created in the Trunk Roads Act of 1936 when the UK Ministry of Transport took direct control over 30 of the principal roads in Great Britain from English, Welsh and Scottish local authorities. The number of trunk roads was increased from 30 to 101 in the Trunk Roads Act of 1946. These roads formed what the Act called "the national system of routes for through traffic". Since Welsh devolution the trunk road system in Wales has been managed by the South Wales Trunk Road Agent and the North and Mid Wales Trunk Road Agent on behalf of the Welsh Government. As of April 2019, out of a total of 34,850 miles (56,090 km) of roads in Wales, 1,576 miles (2,536 km) are trunk roads.

The A40 is a trunk road which runs between London and Goodwick (Fishguard), Wales, and officially called The London to Fishguard Trunk Road (A40) in all legal documents and Acts. Much of its length within England has been superseded by motorways, such as the M40, and has lost its trunk road status, though it retains it west of Gloucester, including its length within Wales. It is approximately 260 miles (420 km) long. The eastern section from Denham, Buckinghamshire to Wheatley, Oxfordshire is better served by the M40 and its former function of linking London with Cheltenham and Gloucester has been taken by the M4, A419 and A417 via Swindon.