A narrowboat is a particular type of canal boat, built to fit the narrow locks of the United Kingdom. The UK's canal system provided a nationwide transport network during the Industrial Revolution, but with the advent of the railways, commercial canal traffic gradually diminished and the last regular long-distance transportation of goods by canal had virtually disappeared by 1970. However, some commercial traffic continued. From the 1970s onward narrowboats were gradually being converted into permanent residences or as holiday lettings. Currently, about 8580 narrowboats are registered as 'permanent homes' on Britain's waterway system and represent a growing alternative community living on semi-permanent moorings or continuously cruising.

For any boat to enter a narrow lock, it must be under 7 feet (2.13 m) wide, so most narrowboats are nominally 6 feet 10 inches (2.08 m) wide. A narrowboat's maximum length is generally 72 feet (21.95 m), as anything longer will be unable to navigate much of the British canal network, because the nominal maximum length of locks is 75 feet (22.86 m). Some locks are shorter than 72 feet (21.95 m), so to access the entire canal network the maximum length is 57 feet (17.37 m).

The first narrow boats played a key role in the economic changes of the British Industrial Revolution. They were wooden boats drawn by a horse walking on the canal towpath led by a crew member. Horses were gradually replaced by steam and then diesel engines. By the end of the 19th century it was common practice to paint roses and castles on narrowboats and their fixtures and fittings. This tradition has continued into the 21st century, but not all narrowboats have such decorations.

Modern narrowboats are used for holidays, weekend breaks, touring, or as permanent or part-time residences. Usually, they have steel hulls and a steel superstructure. The hull's flat base is usually 10mm thick, the hull sides 6mm or 8mm, the cabin sides 6mm, and the roof 4mm or 6mm. The numbers of boats have been rising, with the number of licensed boats (not all of them narrowboats) on canals and rivers managed by the Canal & River Trust (CRT) estimated at about 27,000 in 2006; by 2019, this had risen to 34,367. Although a small number of steel narrowboats dispense with the need for a rear steering deck entirely, by imitating some river cruisers in providing wheel steering from a central cockpit, most narrowboats' steering is by a tiller on the stern. There are three major configurations for the stern: traditional stern, cruiser stern and semi-traditional stern.