Party discipline is the ability of a political party to get its members to support the policies of the party leadership.

In a Western context, it usually refers to the control that party leaders have over its legislative members. Party discipline tends to be extremely strong in Westminster systems such as Canada or India in which a vote by the legislature against the government is understood to cause the government to "collapse," according to the convention of confidence votes. In these situations, it is extremely rare and suicidal for a member to vote against the wishes of their party. Party leaders in such governments often have the authority to fire members who violate the party line. Party discipline is much weaker in Congressional systems such as the United States Congress. In these legislatures, it is routine for members to cross party lines.

Almost all legislative democracies feature party whips, whose job is to enforce party discipline, although the powers of such a position varies considerably from party to party and country to country.

The term has a somewhat different meaning in Marxist-Leninist political systems such as the People's Republic of China. In this case it refers to administrative sanctions such as fines or expulsion that the Communist Party can impose on its members for actions such as corruption or working against the party.