The Westminster system or Westminster model is a type of parliamentary government that incorporates a series of procedures for operating a legislature. This concept was first developed in England.

Key aspects of the system include an executive branch made up of members of the legislature, and that is responsible to the legislature; the presence of parliamentary opposition parties; and a ceremonial head of state who is different from the head of government. The term comes from the Palace of Westminster, the current seat of the Parliament of the United Kingdom. The Westminster system is often contrasted with the presidential system that originated in the United States, or with the semi-presidential system, based on the government of France.

The Westminster system is used, or was once used, in the national and subnational legislatures of most former colonies of the British Empire, upon gaining self-government (with the exception of the United States and Cyprus), beginning with the first of the Canadian provinces in 1848 and the six Australian colonies between 1855 and 1890. It is the form of government bequeathed to New Zealand, and former British Hong Kong. The State of Israel adopted a largely Westminster-inspired system of government upon declaring independence from the British Mandate of Palestine. However, some former colonies have since adopted either the presidential system (Nigeria for example) or a hybrid system (like South Africa) as their form of government.