Cardiff Bay is an area and freshwater lake in Cardiff, Wales. The site of a former tidal bay and estuary, it is the river mouth of the River Taff and Ely. The body of water was converted into a 500-acre (2.0 km2) lake as part of a UK Government redevelopment project, involving the damming of the rivers by the Cardiff Bay Barrage in 1999. The barrage impounds the rivers from the Severn Estuary, providing flood defence and the creation of a permanent non-tidal high water lake with limited access to the sea, serving as a core feature of the redevelopment of the area in the 1990s.

Cardiff is the capital and largest city of Wales. Cardiff had a population of 362,310 in 2021 and forms a principal area officially known as the City and County of Cardiff. The city is the eleventh largest in the United Kingdom. Located in the southeast of Wales and in the Cardiff Capital Region, Cardiff is the county town of the historic county of Glamorgan and in 1974–1996 of South Glamorgan. It belongs to the Eurocities network of the largest European cities. A small town until the early 19th century, its prominence as a port for coal when mining began in the region helped its expansion. In 1905, it was ranked as a city and in 1955 proclaimed capital of Wales. Cardiff Built-up Area covers a larger area outside the county boundary, including the towns of Dinas Powys and Penarth.