Belfast (from Irish: Béal Feirste meaning "mouth of the sandbars") is the capital of and the largest city in Northern Ireland. It is the seat of devolved government and legislative Northern Ireland Assembly. It is the largest urban area in Northern Ireland, the second-largest city in Ireland and the 15th-largest city in the United Kingdom. It is the main settlement in the province of Ulster. The city of Belfast has a population of 267,500 and lies at the heart of the Belfast urban area, which has a population of 483,418. The Belfast metropolitan area has a total population of 579,276. Belfast is also the 100th-largest urban zone in the EU. Belfast was granted city status in 1888.

Historically, Belfast has been a centre for the Irish linen industry (earning the nickname "Linenopolis"), tobacco production, rope-making and shipbuilding: the city's main shipbuilders, Harland and Wolff, which built the ill-fated RMS Titanic, propelled Belfast on to the global stage in the early 20th century as the largest and most productive shipyard in the world. Belfast played a key role in the Industrial Revolution, establishing its place as a global industrial centre until the latter half of the 20th century.

Industrialisation and the inward migration it brought made Belfast, if briefly, the largest city in Ireland at the turn of the 20th century and the city's industrial and economic success was cited by Ulster unionist opponents of Home Rule as a reason why Ireland should shun devolution and later why Ulster in particular would fight to resist it.

Today, Belfast remains a centre for industry, as well as the arts, higher education and business, a legal centre, and is the economic engine of Northern Ireland. The city suffered greatly during the period of disruption, conflict, and destruction called the Troubles, but latterly has undergone a sustained period of calm, free from the intense political violence of former years, and substantial economic and commercial growth. Belfast city centre has undergone considerable expansion and regeneration in recent years, notably around Victoria Square.

Belfast is served by two airports: George Best Belfast City Airport in the city, and Belfast International Airport 15 miles (24 km) west of the city.

Belfast is also a major seaport, with commercial and industrial docks dominating the Belfast Lough shoreline, including the famous Harland and Wolff shipyard.

Belfast is a constituent city of the Dublin-Belfast corridor, which has a population of 3 million, or half the total population of the island of Ireland.