A two-party system is a political party system in which two major political parties consistently dominate the political landscape. At any point in time, one of the two parties typically holds a majority in the legislature and is usually referred to as the majority or governing party while the other is the minority or opposition party. Around the world, the term has different meanings. For example, in the United States, the Bahamas, Jamaica, and Zimbabwe, the sense of two-party system describes an arrangement in which all or nearly all elected officials belong to either of the two major parties, and third parties rarely win any seats in the legislature. In such arrangements, two-party systems are thought to result from several factors, like "winner takes all" or "first past the post" election systems.

In such systems, while chances for third-party candidates winning election to major national office are remote, it is possible for groups within the larger parties, or in opposition to one or both of them, to exert influence on the two major parties. In contrast, in Canada, the United Kingdom and Australia and in other parliamentary systems and elsewhere, the term two-party system is sometimes used to indicate an arrangement in which two major parties dominate elections but in which there are viable third parties or independents that do win some seats in the legislature, and in which the two major parties exert proportionately greater influence than their percentage of votes would suggest.

Explanations for why a political system with free elections may evolve into a two-party system have been debated. A leading theory, referred to as Duverger's law, states that two parties are a natural result of a winner-take-all voting system.