The A12 is a major road in Eastern England. It runs north-east/south-west between London and the coastal town of Lowestoft in the north-eastern corner of Suffolk, following a similar route to the Great Eastern Main Line until Ipswich. A section of the road between Lowestoft and Great Yarmouth became part of the A47 in 2017. Between the junctions with the M25 and the A14, the A12 forms part of the unsigned Euroroute E30 (prior to 1985, it was the E8). Unlike most A roads, this section of the A12, together with the A14 and the A55, has junction numbers as if it were a The 52 miles (84 km) section of the A12 through Essex has sections of dual two lanes and dual three lanes, with eight changes in width between the M25 to Ipswich. It was named as Britain's worst road because of "potholes and regular closures due to roadworks" in a 2007 survey by Cornhill Insurance. The A12 is covered by the National Highways A12 / A120 route based strategy.

Starting just north of the Blackwall Tunnel, where it connects end on to the A102, it heads north through Bow and Hackney Wick, then north-east through Leyton and Romford, then into Essex, passing Brentwood and Colchester. In Suffolk, it passes Ipswich and Saxmundham, then follows the coast through Lowestoft before entering Norfolk, passing through Gorleston-on-Sea and ending at Great Yarmouth. In February 2017, the route was renumbered between Great Yarmouth and Lowestoft to become the A47. In June 2024, most of the A1117 was renumbered as the A12, incorporating the Lowestoft Gull Wing Bridge, where the A12 now terminates at the A47 in North Lowestoft.

The A12 was formed in 1922 as part of the Great Britain road numbering scheme, and initially the route went from Stratford to Gallows Corner along the present A118 road before continuing to Great Yarmouth. This section in London was rerouted to run on Eastern Avenue by the mid-1940s, and extended to follow the current route from Blackwall Tunnel along the East Cross Route, (previously the A102(M) & A106), the M11 link road in 1999.

The route from London to Essex has long been important, with Old Ford being the location of an ancient Celtic crossing of the River Lea. The route was altered slightly by the Romans who created a paved road from London to Colchester, which was part of Inter V on the Antonine Itinerary, and parts of this were used by a turnpike road, the Great Essex Road. The crossing of the Lea moved to its current location at Bow around 1110 when Matilda, wife of Henry I, ordered a distinctively bow-shaped, three-arched bridge to be built over the river. A map from 1766 shows a route from London to Lowestoft which follows much of the current A12.

The "Ipswich to South Town and Bungay Turnpike Trust" was established in 1785, operating between Ipswich and Great Yarmouth. The trust was wound up in 1872 following the arrival of the East Suffolk Line which was fully operational between the two towns in 1859. Following the demise of the Turnpike trust, responsibility reverted to parish councils until the new county councils took over in 1889.

A new section of the A12, known as the "M11 link road" or "A12 Hackney-M11 Link Road", was built in the early 1990s in the face of the major M11 link road protest and finally opened in October 1999. The section of road had originally been proposed in 1903 in a Royal Commission on London Traffic. A public inquiry had been held in September 1961 and a further three public inquiries, a Parliamentary Bill and a High Court challenge had been required before the work started.

Initiated in 2000, the London to Ipswich Multi-modal study reported its conclusions late in 2002.

In 2008 improvements were made to the junction between the A12 and the M25 to increase slip-road capacity, in particular for clockwise M25 traffic turning north onto the A12, and to ease congestion on the Brook Street Roundabout (serving the M25, A12 and local Brentwood traffic as the A1023).