Hadrian's Wall (Latin: Vallum Hadriani, also known as the Roman Wall, Picts' Wall, or Vallum Aelium in Latin) is a former defensive fortification of the Roman province of Britannia, begun in AD 122 in the reign of the Emperor Hadrian. Running from Wallsend on the River Tyne in the east to Bowness-on-Solway in the west of what is now northern England, it was a stone wall with large ditches in front of it and behind it that crossed the whole width of the island. Soldiers were garrisoned along the line of the wall in large forts, smaller milecastles, and intervening turrets. In addition to the wall's defensive military role, its gates may have been customs posts.

Hadrian's Wall Path generally runs very close to the wall. Almost all of the standing masonry of the wall was removed in early modern times and used for local roads and farmhouses. None of it stands to its original height, but modern work has exposed much of the footings, and some segments display a few courses of modern masonry reconstruction. Many of the excavated forts on or near the wall are open to the public, and various nearby museums present its history. The largest Roman archaeological feature in Britain, it runs a total of 73 miles (117.5 kilometres) in northern England. Regarded as a British cultural icon, Hadrian's Wall is one of Britain's major ancient tourist attractions. It was designated as a UNESCO World Heritage Site in 1987. The turf-built Antonine Wall in what is now central Scotland, which briefly superseded Hadrian's Wall before being abandoned, was declared a World Heritage Site in 2008.

Hadrian's Wall marked the boundary between Roman Britannia and unconquered Caledonia to the north. The wall lies entirely within England and has never formed the Anglo-Scottish border, though it is sometimes loosely or colloquially described as being such.