The A5 is a major road in the United Kingdom. It runs from London to Holyhead, on Anglesey in Wales, following in part the route of the Roman Watling Street. It was constructed by Thomas Telford and large stretches of the road in Wales remain much as it was originally built with many of the toll houses still present. It was designed to allow stage coaches to post between London and Holyhead, and thence to Ireland, and throughout its length the gradient never excceds 5%.

Starting at Marble Arch, the A5 runs north-west up the Edgware Road. The A5 as numbered disappears near Edgware, but re-emerges south of Luton. However, the road itself continues as the A5183 from Edgware to Elstree, Borehamwood, Radlett, St Albans and Redbourn, to junction 9 of the M1, where it becomes numbered as the A5 again. It then travels through Dunstable, Milton Keynes and Towcester, near Rugby and Hinckley, then through Tamworth. Passing through urban Brownhills, the road is intersected by roundabouts and slip roads for the M6 Toll motorway before it continues onto via Cannock to Telford. After bypassing Shrewsbury and Oswestry and Nesscliffe, it enters Wales just west of Chirk and continues through Llangollen and Betws-y-Coed and then ascends through Capel Curig and reaches its maximum height at 312 metres shortly before passing Llyn Ogwen. The road then descends between some of Snowdonia's greatest peaks before passing through Bethesda and through Bangor before crossing the Menai Strait on the Menai Suspension Bridge near Bangor. The A5 then crosses Anglesey in parallel to the A55 before arriving at the port of Holyhead, where it has traditionally ended at Admiralty Arch.

Parts of the A5 have been superseded by sections of the M1, M54 through Telford and M6 Toll motorways.

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There is also an A5 road in Northern Ireland. It runs from Derry south, via Strabane and Omagh, to join the N2 road at the border with the Republic of Ireland.