Hadrian's Wall 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 (in its final version) with large ditches in front and behind, stretching across 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 close along the wall. Almost all 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). Regarded as a British cultural icon, Hadrian's Wall is one of Britain's major ancient tourist attractions. It was designated a UNESCO World Heritage Site in 1987. The turf-built Antonine Wall of AD 142 in what is now central Scotland, which briefly superseded Hadrian's Wall before being abandoned, was declared a World Heritage Site in 2008.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 lies entirely within England and has never formed the Anglo-Scottish border, though it is sometimes loosely or colloquially described as such.

The length of the wall was 80 Roman miles, equivalent to 73 modern miles; or 117 kilometres (1 Roman mile is equivalent to 1,620 yards; or 1,480 metres). This traversed the entire width of the island, from Bowness-on-Solway in the west to Wallsend on the River Tyne in the east. Not long after construction began, the wall's thickness was reduced from the originally planned 10 feet (3.0 m) to about 8 feet (2.4 m), or even less depending on the terrain. Some sections were originally constructed of turf and timber, eventually replaced by stone years or decades later.

None of the wall stands to its original height. Bede, a monk and historian who died in 735, wrote that the wall stood 12 feet (3.7 metres) high, with evidence suggesting it could have been a few feet higher at its formation. Along the length of the wall there was a watch-tower turret every third of a mile, also providing shelter and living accommodation for Roman troops.

Hadrian's Wall extended west from Segedunum at Wallsend on the River Tyne, via Carlisle and Kirkandrews-on-Eden, to the shore of the Solway Firth, ending a short but unknown distance west of the village of Bowness-on-Solway. The route was slightly north of Stanegate, an important Roman road built several decades earlier to link two forts that guarded important river crossings: Corstopitum (Corbridge) on the River Tyne and Luguvalium (Carlisle) on the River Eden. The modern A69 and B6318 roads follow the course of the wall from Newcastle upon Tyne to Carlisle, then along the northern coast of Cumbria (south shore of the Solway Firth).

Part of the central section of the wall follows natural cliffs on an escarpment of the Whin Sill rock formation.

Although the curtain wall ends near Bowness-on-Solway, this does not mark the end of the line of defensive structures. The system of milecastles and turrets is known to have continued along the Cumbria coast as far as Risehow, south of Maryport. For classification purposes, the milecastles west of Bowness-on-Solway are referred to as Milefortlets.