The Second Severn Crossing (Welsh: Ail Groesfan Hafren), officially named the Prince of Wales Bridge (Welsh: Pont Tywysog Cymru) since July 2018, is the M4 motorway bridge over the River Severn between England and Wales, opened in 1996 to supplement the traffic capacity of the Severn Bridge built in 1966. The bridge is operated by England's National Highways. It has a total length of 5,128 metres (16,824 ft).

It is southwest of the Severn Bridge and because it is more closely in line with the rest of the M4, it reduces the length of the journey between England and Wales. The junctions at each end are designed for most traffic to use this crossing, and in order to use the old Severn Bridge, one has to leave the M4 at junction 21 and join the M48 near Aust or at junction 23 near Magor. The new crossing carries more traffic than the Severn Bridge, which is still in use. It is wider than the Severn Bridge, having three lanes and a narrow hard shoulder each way, compared to the two lanes, cycle path and narrow footpath of the original crossing. It is a cable-stayed bridge, whereas the Severn Bridge is a suspension bridge. The position of the bridge is close to that of the Severn Tunnel, which has carried the railway line beneath the river bed since 1886.

Much of the estuary is mudflats at low tide, but at high tide these can be covered by as much as 14 metres (46 ft) of water. This presented the engineers with a constraint: packets of work were scheduled at low tide, and needed to be completed within the short windows allowed by the tides.

The concession given to the consortium which financed, built and operate the bridge required them to take over the outstanding debt on the original Severn Bridge and to operate the two bridges as a single entity. Tolls were set annually by the government based on the previous year's change in the Retail Price Index. On the expiry of the concession in January 2018, the consortium was required to hand the bridge over to public ownership. The tolls on both bridges were subsequently scrapped in December 2018.

The Severn Estuary presented a barrier between the Bristol area and South Wales. The estuary has a maximum tidal range of 14.5 metres (48 ft), amongst the highest in the world, and during a rising or falling tide there are strong currents of up to 8 knots (4.1 m/s). Much of the estuary is mud flats that are exposed at low tide; these have been designated a Special Protection Area. The central part of the estuary is a navigable channel which, at the site of the bridge, is known as "The Shoots". The bridge is upstream from Avonmouth and the Port of Bristol, but downstream from the Port of Sharpness. The Gloucester Harbour Trustees have responsibility for controlling navigation in the estuary's tidal waters upstream from the bridge.

Until 1966, road travellers could either use the Aust Ferry, which had operated since medieval times (and as a car ferry since 1926), or use the scheduled car shuttle train service through the Severn Tunnel between Pilning and Severn Tunnel Junction, which operated from 1926 until 1966. Otherwise, the detour via Gloucester would add 57 miles (92 km) to the journey. In 1966, the first Severn road bridge, a four-lane suspension bridge, was opened, carrying the M4 motorway between England and South Wales. By 1984, traffic across the first Severn Bridge had tripled and it was projected that by the mid-1990s, the old bridge would be running at capacity. A study was commissioned into building either a second bridge or a tunnel. The consultants reported back in 1986, recommending that a new bridge be built downstream from the existing bridge.

In 1988, it was announced that tenders would be invited from private consortia to fund, build and operate the bridge for a specified period. The consortium would also take over the management of the old bridge and the associated £100 million debt. Tenders were invited in 1989; and in 1990 the concession to build the bridge was awarded to Severn River Crossing plc. Construction work started on 26 April 1992 and the bridge was opened by Prince Charles (the future Charles III, then the Prince of Wales), on 5 June 1996.

The chief architect of the bridge was Ronald Weeks of the Cardiff-based Percy Thomas Partnership, with the detailed engineering design by the Halcrow Group and the French consultancy SEEE [fr]. The bridge has portals close to Sudbrook, Monmouthshire on the Welsh side and Severn Beach in South Gloucestershire on the English side. It has three principal sections – a 25 span viaduct on the English side of length 2,103 m (6,900 ft; 1.307 mi), a 24 span viaduct of length 2,077 m (6,814 ft; 1.291 mi) on the Welsh side and the bridge itself, a 948 m (3,110 ft; 0.589 mi) structure with a 37 m (121 ft) navigational clearance, giving a total length of 5,128 m (16,824 ft; 3.186 mi). The central section, called the Shoots Bridge, is of cable-stayed design and the central span (between the bridge pylons) is 456 metres (1,496 ft) in length. The approach viaducts are of a segmental bridge design. The crossing forms a very slight reverse "S" curve – the roadway has an approximate east–west alignment at each of the portals, while the central bridge follows an alignment approximately WNW to ESE.

The Severn Railway Tunnel passes under the estuary bed on a line which is generally about 500 metres (1,600 ft) upstream of the bridge, but which passes under the line of the bridge close to the English shore.

The deck, which carries three lanes of traffic in each direction, is 34.6 metres (114 ft) wide. The sides of the bridge are fitted with 3-metre (10 ft) baffle plates to reduce lateral wind loads coming from the Severn Estuary onto the traffic; this has reduced the number of times that speed restrictions have been needed. The overall design of the new crossing makes it more resistant to high winds than the old Severn Bridge.

A small diesel-powered monorail, the Rapid Access Train, runs underneath the full length of the crossing at a top speed of 1.5 metres per second (3.4 mph; 5.4 km/h), and is used for safe access to ten stations, such as lifts inside the cable pylons, for bridge maintenance. It can carry up to four passengers plus two tonnes of equipment in its trailer, with an integral crane and safety features such as interlocks, emergency battery propulsion capable of returning to either end of the crossing, and a dead man's switch. During the 1990s and early 2000s, the Rapid Access Train was initially disused for a number of years due to rail faults, but has subsequently been returned to service. Travelling access gantries are also present underneath each segment of the crossing to allow for inspection and maintenance across the full width of the structure, with hydraulic lift platforms capable of reaching multiple levels.

The crossing was built by a business consortium under a public-private partnership. A company called Severn River Crossing plc, led by John Laing plc and GTM-Entrepose, was formed to build the new crossing. This company also took over the responsibility of managing and maintaining the old Severn Bridge crossing, as well as managing and maintaining the new crossing. The cost of constructing the new crossing was to be paid for by tolls collected from motorists using the two crossings. Work on the new crossing began in 1992. Completion was in 1996.

Sub-assemblies for the bridge were constructed onshore and then shifted by a large tracked vehicle (similar to that used to move the Apollo and Space Shuttle at Cape Kennedy) onto a barge (the SAR3), prior to being floated out on the high tide to the site. The 37 bridge pier foundations on the approach viaducts are 98.11 metres (321.9 ft) apart, and consist of open concrete caissons weighing up to 2,000 tonnes, which were founded on the rock of the estuary bed. The decking consists of 3.6-metre (11.8 ft) post-stressed match cast sections, weighing 200 tonnes (200 long tons; 220 short tons) each.

The cable-stayed section of the crossing is over 1,044.7 metres (3,427.5 ft) long, consisting of a 34.6-metre (113.5 ft) wide deck made from steel plate girders with a composite reinforced concrete slab. These were prefabricated on shore and put in place using balanced cantilever methods. There are two high twin-leg, reinforced and pre-stressed concrete pylons carrying 240 cables which support the bridge deck, rising to a height of 149 metres (489 ft) above the river bed or 101 metres (331 ft) above the bridge deck. Cable vibrations were experienced during construction and secondary cables were added to eliminate this. To avoid detracting from the aesthetics of the primary cables, the secondary cables are very slender and are not very noticeable.

During the summer of 1994 the bridge deck launching gantry fell onto the Gwent viaduct, causing the 200-tonne (200-long-ton; 220-short-ton) bridge deck unit to fall onto the deck below. Although this delayed the project by ten weeks, the builders still completed the bridge on schedule two years later.

The crossing passes over mudflats in the Severn Estuary with part of the eastern approach viaduct sited on the English Stones, a rocky outcrop uncovered at low tide. The estuary wetlands are home to migrating birds such as the ringed plover, redshank and whimbrel, while the curlew, dunlin and grey plover winter in the area. The birds feed on ragworm, lugworm and other invertebrates. Saltmarsh is found along the fringes of the coast. Beds of eelgrass occur on the more sheltered mud- and sandbanks. In 1976, in recognition of the importance of the estuary as a wetland used by migrating birds, an area of 247 km (95 sq mi) was designated a Ramsar site. In 1988, the Severn Estuary was designated a Special Protection Area. The same year an area of 99 km (38 sq mi) was notified as a Site of Special Scientific Interest and designated as such the following year; in 1995, this was extended to cover the whole of the Ramsar site.

Prior to the bridge's construction, environmentalists raised numerous concerns and criticisms, chiefly about the immediate damage from construction work and the effects of long-term pollution from a projected increase in car traffic. The construction process resulted in a temporary increase in turbidity of the Severn waters. Although eelgrass is reasonably tolerant to short-term high turbidity and consequent loss of light, the bed of eelgrass in the river was observed to decline considerably during the period of construction. The construction of the approach roads and toll plaza resulted in the permanent loss of some wet pastureland.

Field surveys carried out in 2003 and 2004 reported that the bridge had little effect on the flight paths of the various birds – it was noted that when curlews, dunlins and lapwings approached the bridge, they would change course to gain height and fly over the bridge, but that oystercatchers and turnstones would fly under the bridge. The surveys also showed that the bridge had minimal impact on the roosting habits of most birds, though a flock of mallards were seen to be roosting directly under the bridge, with many positioning themselves on the dry concrete base on one of the pillars. Likewise, it was noted that the bridge had little impact on the birds' feeding habits – turnstones and lapwings were recorded as feeding on mudflats directly under the bridge while the absence of other waders was attributed to the rocky nature of the foreshore.

The consortium consisting of two civil engineering firms and two banks that funded and built the bridge had a 30-year concession to redeem its outlay from tolls collected from users of the bridge. At the end of the concession period in January 2018, the bridge passed into public ownership, and is now managed by National Highways.