A single-party state, one-party system or single-party system is a type of party system government in which a single political party forms the government and no other parties are permitted to run candidates for election.

Sometimes the term de facto single-party state is used to describe a dominant-party system where laws or practices prevent the opposition from legally getting power.

Typically, single-party states hold the suppression of political factions, except as transitory issue oriented currents within the single party or permanent coalition as a self evident good.

Some single party states only outlaw opposition parties, while allowing subordinate allied parties to exist as part of a permanent coalition such as a popular front. Within their own countries, dominant parties ruling over single-party states are often referred to simply as the Party. For example, in reference to the Soviet Union, the Party meant the Communist Party of the Soviet Union; in reference to the former People's Republic of Poland it referred to the Polish United Workers' Party.