The Menai Strait (in Welsh Afon Menai, the "River Menai") 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.

The strait is bridged in two places - the main A5 road 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 rectangular box spans, but after a disastrous fire in 1970, which left only the limestone pillars remaining, it was rebuilt as a steel box girder bridge.

In places the strait is nearly two miles acroos but it narrows to little more than 200 metres where it passes between the two bridges. 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. One of the most dangerous areas of the Strait is known as the Swellies (or Swillies) between the two bridges. Here rocks near the surface cause over-falls and local whirlpools which can be of considerable danger in themselves and cause small boats to founder on the rocks. This was the site of the loss of the school ship HMS Conway in 1953. Entering the strait at the Caernarfon end is also hazardous because of the frequently shifting sand banks that make up Caernarfon bar. Entering the strait at the south western end shipping has to pass Fort Belan an 18th century defensive fort built in the times of the American War of Independence.

The observed tidal effects observed along the banks of the strait can also be confusing. A rising tide approaches from the south-west and cause the water in the strait to flow north-eastwards as the level rises. The tide also flows around Anglesey until, after a few hours, it starts to flow into the strait in a south-easterley direction from Beaumaris. By the time this happens the tidal flow from the Caernarfon end is weakening and the tide continues to rise but the direction of tidal flow is reversed. A similar sequence is seen in reverse on a falling tide. This means that slack water between the bridges tends to occur approximately one hour before high tide or low tide.

At slack water low tide, it is possible to cross the strait at Lafan Sands (Traeth Lafan), northeast of Bangor, but this is not advisable without good local knowledge.

Because the strait has such unusual tidal conditions coupled with very low wave heights because of its sheltered position, it present a unique and diverse benthic ecology. The existence of this unique ecology was a major factor in the establishment of the internationally renowned School of Ocean Sciences at Menai Bridge, part of the University of Bangor.

Much of the land on Anglesey at the eastern end of the strait is designated as an area of outstanding natural beauty (AONB). Despite this designation there have a number of decisions by planning authorities that do not reflect this designation. The construction of the flats at Glyn Garth is perhaps the best known exampe of the creation of a blot on this landscape.

Places bordering the strait include:

• Beaumaris
• Bangor
• Caernarfon
• Dwyran
• Menai Bridge
• Y Felinheli, in English Port Dinorwic

Just north of the rail bridge on Anglesey is the famously named Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch.