A parliamentary system, or parliamentarian democracy, is a system of democratic governance of a state (or subordinate entity) where the executive derives its democratic legitimacy from its ability to command the support ("confidence") of the legislature, typically a parliament, to which it is accountable. In a parliamentary system, the head of state is usually a person distinct from the head of government. This is in contrast to a presidential system, where the head of state often is also the head of government and, most importantly, where the executive does not derive its democratic legitimacy from the legislature.

Countries with parliamentary systems may be constitutional monarchies, where a monarch is the head of state while the head of government is almost always a member of parliament, or parliamentary republics, where a mostly ceremonial president is the head of state while the head of government is regularly from the legislature. In a few parliamentary republics, among some others, the head of government is also head of state, but is elected by and is answerable to parliament. In bicameral parliaments, the head of government is generally, though not always, a member of the lower house.

Parliamentarianism is the dominant form of government in Europe, with 32 of its 50 sovereign states being parliamentarian. It is also common in the Caribbean, being the form of government of 10 of its 13 island states, and in Oceania. Elsewhere in the world, parliamentary countries are less common, but they are distributed through all continents, most often in former colonies of the Malay states that subscribe to a particular brand of parliamentarianism known as the Westminster system.