Belfast ( BEL-fast, -⁠fahst; from Irish: Béal Feirste [bʲeːlˠ ˈfʲɛɾˠ(ə)ʃtʲə], meaning 'mouth of the sand-bank ford') is the capital and largest city of Northern Ireland, standing on the banks of the River Lagan on the east coast. It is the 10th-largest primary urban area in the United Kingdom and the second-largest city in Ireland. It had a population of 345,418 in 2021.

By the early 19th century, Belfast was a major port. It played an important role in the Industrial Revolution in Ireland, briefly becoming the biggest linen-producer in the world, earning it the nickname "Linenopolis". By the time it was granted city status in 1888, it was a major centre of Irish linen production, tobacco-processing and rope-making. Shipbuilding was also a key industry; the Harland & Wolff shipyard, which built the RMS Titanic and SS Canberra, was the world's largest shipyard. Industrialisation, and the resulting inward migration, made Belfast one of Ireland's biggest cities. Following the partition of Ireland in 1921, Belfast became the seat of government for Northern Ireland. There was major communal violence in the city during partition. Belfast saw further severe violence and numerous bombings during the thirty years of the Troubles, c. 1969–1998, and parts of the city remain segregated between Catholics and Protestants.

Belfast is still a port with commercial and industrial docks, including the Harland & Wolff shipyard, dominating the Belfast Lough shoreline. It also has a major aerospace industry. It is served by two airports: George Best Belfast City Airport, 3 miles (5 kilometres) from the city centre, and Belfast International Airport 15 miles (24 kilometres) west of the city. The Globalization and World Cities Research Network listed Belfast as a Gamma + global city in 2020.