arrangement requiring workers to join a particular union and pay dues within a specified period of time after beginning employment—usually 30 to 90 days. Such an arrangement guarantees that workers will pay for the benefits of union representation. A union shop is less restrictive than a closed shop, which prevents employers from hiring outside the union.

In most countries, union shop agreements are uncommon because one union seldom gains exclusive bargaining rights for all of a particular employer's workers. In Japan, where a single union does customarily represent all the employees in a company, union shop agreements are both legal and common. (See enterprise unionism.) In the United States, a single union may be chosen by majority vote to represent all the workers; however, under Section 14(b) of the Taft-Hartley Act (Taft–Hartley Act), a state may outlaw union shop provisions in labour contracts by passing right-to-work laws (right-to-work law), which prohibit requiring union membership as a condition of employment.

The status of a union shop may also be challenged by its members. This happens when a majority of union employees vote to terminate the union shop provision in their contract—thus removing a union's most desired form of security. Lacking a union shop or a closed shop, workplaces are defined as either agency shops (which require employees to contribute funds equal to union dues but not join the union) or open shops (which require neither membership nor dues payment). Employees in open shops who benefit from the gains that unions achieve through collective bargaining, without sharing the expenses, are sometimes called “free riders.”