A single-party state or 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 unfair laws or practices prevent the opposition from legally getting power. 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.

A one-party system should not be confused with a non-partisan democracy which prohibits all political parties. Also, some one-party states may allow non-party members to run for legislative seats, as was the case with Taiwan's Tangwai movement in the 1970s and 1980s.

In most cases, single-party states have arisen from Leninist, fascist or nationalist ideologies, particularly in the wake of independence from colonial rule. One-party systems often arise from decolonization because one party has had an overwhelmingly dominant role in liberation or in independence struggles.

Where the ruling party subscribes to a form of Marxism-Leninism, the one-party state system is usually called a communist state, though such states do not use that term to describe themselves, adopting instead the title of people's republic, socialist republic or democratic republic. In communist states such as Cuba, the role of the Communist Party is enshrined in the constitution, and no party is permitted to campaign or run candidates for election, including the Communist party. Candidates are elected on an individual referendum basis without formal party involvement, though elected assemblies predominantly consist of members of the dominant party alongside non-affiliated candidates.