The United Kingdom has three distinctly different legal systems, each of which derives from a particular geographical area for a variety of historical reasons: English law (in the joint jurisdiction of England and Wales), Scots law, Northern Ireland law, and, since 2007, calls for a fourth type, that of purely Welsh law as a result of Welsh devolution, with further calls for a Welsh justice system.

In fulfilment of its former EU treaty obligations, European Union directives had been transposed into the UK legal system on an ongoing basis by the UK parliament. Upon Brexit, non-transposed EU law (such as regulations) was transplanted into domestic law as "retained EU law", with an additional period of alignment with EU law during the transition period from 31 January to 31 December 2020.

There are three distinct legal jurisdictions in the United Kingdom: England and Wales, Northern Ireland and Scotland. Each has its own legal system, distinct history and origins, although there is a substantial overlap between these three legal systems and the three legal jurisdictions.

Unlike the other three, Welsh law is not a separate legal system per se, merely the primary and secondary legislation generated by the Senedd, interpreted in accordance with the doctrines of English law and not impacting upon English common law (except where such Welsh legislation ousts a common law rule by virtue of being a superior form of law).

The UK does not have a single legal system because it was created by the political union of previously independent countries. Article 19 of the Treaty of Union, put into effect by the Acts of Union in 1707, created the Kingdom of Great Britain but guaranteed the continued existence of Scotland's and England's separate legal systems. The Acts of Union of 1800, which joined Great Britain and Ireland into the United Kingdom of Great Britain and Ireland, contained no equivalent provisions but preserved the principle of different courts to be held in Ireland, of which the part called Northern Ireland continues to follow as part of the United Kingdom.

Each legal system defaults to its jurisdiction, each of whose courts further that law through jurisprudence. Choice of which jurisdiction's law to use is possible in private law: for example, a company in Edinburgh, Scotland and a company in Belfast, Northern Ireland are free to contract using English law. This is not so in public law (for example, criminal law), where there are set rules of procedure in each jurisdiction.

Although Scotland and Northern Ireland form part of the United Kingdom and share Westminster as a primary legislature, they have separate legal systems. (Even though Scotland became part of the UK over 300 years ago, Scots law has remained remarkably distinct from English law). The UK's highest civil appeal court is the Supreme Court of the United Kingdom, whose decisions are binding on all three UK jurisdictions, as in Donoghue v Stevenson, a Scots case that forms the basis of the UK's law of negligence.

"Great Britain" means England, Wales, Scotland, their adjacent territorial waters and the islands of Orkney and Shetland, the Hebrides and, by virtue of the Island of Rockall Act 1972, Rockall. "United Kingdom" means Great Britain and Northern Ireland and their adjacent territorial waters, but not the Isle of Man, nor the Channel Islands, whose independent status was discussed in Rover International Ltd. v Canon Film Sales Ltd. (1987) and Chloride Industrial Batteries Ltd. v F. & W. Freight Ltd. (1989). "British Islands" – but not "British Isles" – means the United Kingdom, the Isle of Man and the Channel Islands.