The Millennium Stadium (Welsh: Stadiwm y Mileniwm), known since 2016 as the Principality Stadium (Welsh: Stadiwm Principality) for sponsorship reasons, is the national stadium of Wales. Located in Cardiff, it has a retractable roof and is the home of the Wales national rugby union team; it has also held Wales national football team games. Initially built to host the 1999 Rugby World Cup and replacing the National Stadium, it has gone on to host many other large-scale events, such as the Tsunami Relief Cardiff concert, the Super Special Stage of Wales Rally Great Britain, the Speedway Grand Prix of Great Britain and various concerts. It also hosted FA Cup, League Cup and Football League play-off finals while Wembley Stadium was being redeveloped between 2001 and 2006, as well as football matches during the 2012 Summer Olympics.

The stadium is owned by Millennium Stadium plc, a subsidiary company of the Welsh Rugby Union (WRU). The architects were Bligh Lobb Sports Architecture. The structural engineers were WS Atkins and the building contractor was Laing. The total construction cost of the stadium was £121 million, of which the Millennium Commission funded £46 million.

The Millennium Stadium opened in June 1999 and its first major event was an international rugby union match on 26 June 1999, when Wales beat South Africa in a test match by 29–19 before a crowd of 29,000. With a total seating capacity of 73,931, it is the largest stadium in Wales and the fourth largest (and second largest outside London) in the United Kingdom by total capacity. In addition, it is the third-largest stadium in the Six Nations Championship behind the Stade de France and Twickenham. It is also the second-largest stadium in the world with a fully retractable roof and was the second stadium in Europe to have this feature. Listed as a category four stadium by UEFA, the stadium was chosen as the venue for the 2017 UEFA Champions League Final, which took place on 3 June 2017 and was won by Real Madrid. In 2015, the Welsh Rugby Union announced a 10-year sponsorship deal with the Principality Building Society that saw the stadium renamed as the "Principality Stadium" from early 2016.

Until 1969, Cardiff RFC and Wales both played their home matches on the same pitch at Cardiff Arms Park, but all this changed in the 1969–70 season. As a result of an agreement between Cardiff Athletic Club and the WRU, the National Stadium project established that a new stadium for international matches and events was required, with Cardiff RFC moving to a new, purpose-built stadium on the original cricket ground at the site of the former Cardiff Arms Park stadium. By 7 April 1984 the National Stadium was officially opened. However, in 1994, a committee was set up to consider redeveloping the National Stadium, and by 1995 the WRU had been chosen to host the 1999 Rugby World Cup.

In 1995, the National Stadium, which was designed in 1962, only had a capacity of 53,000; other nations' stadia, such as Twickenham (England) with a capacity of 75,000, and Murrayfield Stadium (Scotland) with a capacity of 67,000, had overtaken it. France was also about to build the Stade de France, which would have a capacity of more than 80,000 for the 1998 FIFA World Cup. The original capacity of the National Stadium was 65,000, but this had been reduced to 53,000, due to the Taylor Report. 11,000 of 53,000 capacity was on the East Terrace and the conversion to an all-seater stadium would have reduced the stadium capacity still further to just 47,500.

In addition to the problems of capacity, the National Stadium was also very well hidden by the neighbouring buildings to the south in Park Street, Wood Street and to the east in Westgate Street, and also by Cardiff Rugby Ground in the north. It was only fully visible from across the River Taff in the west. Access to the ground was also very restricted with the main entrance being a narrow opening in Westgate Street to the east which was shared by both vehicles and spectators alike.

The options for the new stadium included adding a third tier to the existing National Stadium, or moving to a new site. This last option was discounted because it would have required a vast car parking facility, and that would have put severe short-term pressures on the local transport infrastructure, creating traffic jams and pollution. The committee eventually chose a new stadium on the same site but with considerable increase in its capacity. It would also involve moving the alignment of the stadium from west–east to north–south. This was the option supported by the Millennium Commission. It would become the fourth redevelopment of the Cardiff Arms Park site. It was also decided that the new stadium should have a sliding roof to accommodate a multi-use venue, with a grass pitch for rugby and football. The only other sliding roofs in Europe at the time were at two Dutch stadia – the Amsterdam Arena, completed in 1996 with a capacity of 50,000; and Gelredome in Arnhem, a 30,000-capacity ground built from 1996 to 1998.

To remain on the Arms Park site, additional space had to be found to allow safe access and to provide room for the increased capacity and improved facilities. This was achieved by the purchase of adjacent buildings to the south and east and by the construction of a new £6 million River Walk by the River Taff on the western side of the stadium.