The Menai Strait is a narrow stretch of shallow tidal water about 14 miles (23 km) long, which separates the island of Anglesey from the mainland of Wales. In places the strait narrows to little more than 200 metres. The differential tides at the two ends of the strait cause very strong currents to flow in both directions through the strait at different times, creating dangerous conditions, though at low, slack tide, it is possible to cross the strait at Lafan Sands, northeast of Bangor. Without good local knowledge it would be foolish to do so however.

The strait is bridged in two places - the main A5 (Watling Street) is carried over the strait by Thomas Telford's elegant iron suspension bridge, the first of its kind, opened in January 1826. Adjacent to this is Robert Stephenson's 1850 Britannia Bridge. Originally this carried rail traffic in two wrought iron tubes, but after a disastrous fire in 1970, which left only the limestone pillars remaining, it was rebuilt as a steel box girder bridge.

Places bordering the strait include:

Just north of the rail bridge on Anglesey is the famously named Llanfairpwllgwyngyllgogerychwyrndrobwll-llantysiliogogogoch.