.uk is the Internet country code top-level domain (ccTLD) for the United Kingdom. Today it is the fifth most popular top-level domain worldwide (after .com, .de, .net and .cn), with over 6 million registrations

The use of .uk rather than .gb for the top-level domain is due to its pre-existing use in the (now obsolete) JANET Name Registration Scheme in which the order of address components were reversed. .uk was made available in DNS to simplify the translation to and from these addresses. There were plans for eventual transition to .gb to accord with correct usage, but this never occurred; the use of .uk is now entrenched as it is impossible to register a .gb domain.

As with other ccTLDs in the early days it was originally delegated to an individual by Jon Postel. In time, it passed to Dr Willie Black at the UK Education and Research Networking Association (UKERNA). Originally, Domain requests were emailed, manually screened by and then forwarded to the UK Naming Committee before being processed by UKERNA. This Committee was in practice a mailing list of representatives of all organisations involved in the UK Internet system at the time.

By the mid-1990s the growth of the Internet, and particularly the advent of the World Wide Web was pushing requests for domain name registrations up to levels that were not manageable by a group of part-time voluntary managers. Oliver Smith of Demon Internet forced the issue by providing the committee with a series of automated tools, called the "automaton", which formalised and automated the naming process end to end. This allowed many more registrations to be processed far more reliably and rapidly, and inspired individuals such as Ivan Pope to explore more entrepreneurial approaches to registration.

Various plans were put forward for the possible management of the domain, mostly Internet service providers seeking to stake a claim, each of which were naturally unacceptable to the rest of the committee. In response to this Dr Black, as the .uk Name, stepped up with a bold proposal for a not-for-profit commercial entity to deal with the .uk domain properly. Commercial interests initially balked at this, but with widespread support Nominet UK was formed to be the .uk Network Information Centre, a role which it continues to this day.

The general form of the rules (i.e. which domains can be registered and whether to allow second level domains) was set by the Naming Committee. Nominet has not made major changes to the rules, although it has introduced a new second level domain .me.uk for individuals.

It is prohibited to register a domain name directly under .uk (such as internet.uk) and a second-level domain must be used (such as internet.co.uk).

However, some domains delegated before the creation of Nominet UK remain. Examples include parliament.uk (Parliament), bl.uk and british-library.uk (the British Library), nls.uk (the National Library of Scotland), nhs.uk (The National Health Service), and jet.uk (UKAEA as operator of the Joint European Torus experimental fusion tokamak). No new 'normal' registrations at the second level are accepted although there is a system for allocating new second level domains to expand the capacity of the system. Such allocations are rarely made.

It is possible to directly register a domain name with Nominet UK but it is faster and cheaper to do it via a Nominet tag holder.