Transmission belt is a Marxist–Leninist analogy to describe interactions of the communist party with the people in a communist state via mass organizations, such as trade unions. All these institutions worked under the party's leadership. Examples are the All-Union Central Council of Trade Unions of the Soviet Union and the All-China Federation of Trade Unions, were and are transmission belt organisations.
The term originates from Vladimir Lenin's speech to the 8th All-Russian Congress of Soviets, the All-Russian Central Council of Trade Unions and the Moscow City Council of Trade Unions, on 30 December 1920.