Cardiff Blues (Welsh: Gleision Caerdydd) are one of the four professional Welsh regional rugby union teams. Based in Cardiff, the capital of Wales, the team have played at Cardiff City Stadium since the start of the 2009/2010 season and are owned by Cardiff Rugby Football Club.

Cardiff Blues are responsible for developing rugby union in the city of Cardiff, Vale of Glamorgan, Rhondda Cynon Taf, Merthyr Tydfil and south Powys. Cardiff Blues are affiliated with a number of semi-professional and amateur clubs throughout the area, including Welsh Premier Division sides Cardiff RFC and Pontypridd RFC.

The Cardiff Blues compete in the Pro12 league, which includes teams from the Celtic nations of Ireland, Scotland and Wales, as well as Italy. In addition, Cardiff Blues compete in the Anglo-Welsh Cup and the Heineken Cup. The Cardiff Blues have qualified for the Heineken Cup every season, and won the 2008-09 Anglo-Welsh Cup and the 2009–10 European Challenge Cup.

The teams main sponsor is currently EADS. The Cardiff Blues kit is manufactured by Canterbury of New Zealand.

Cardiff (ⁱ/ˈkɑrdɪf/; Welsh: Caerdydd (info) Welsh pronunciation: [kairˈdiːð, kaˑɨrˈdɨːð]) is the capital and largest city in Wales and the 10th largest city in the United Kingdom. The city is Wales' chief commercial centre, the base for most national cultural and sporting institutions, the Welsh national media, and the seat of the National Assembly for Wales. The unitary authority area's mid 2010 population was estimated to be 341,054, while the population of the Larger Urban Zone was estimated at 861,400 in 2009. Cardiff is a significant tourism centre and the most popular visitor destination in Wales with 18.3 million visitors in 2010. In 2011, Cardiff was ranked 6th in the world in National Geographic's alternative tourist destinations.

The city of Cardiff is the county town of the historic county of Glamorgan (and later South Glamorgan). Cardiff is part of the Eurocities network of the largest European cities. The Cardiff Urban Area covers a slightly larger area outside of the county boundary, and includes the towns of Dinas Powys and Penarth. A small town until the early 19th century, its prominence as a major port for the transport of coal following the arrival of industry in the region contributed to its rise as a major city.