WASHINGTON—Contract workers and other temporary employees will be able to more easily unionize following a landmark ruling Thursday by a U.S. federal labor regulator.

The ruling from the National Labor Relations Board will ripple through the fast-food, construction and other industries that rely heavily on contract workers and employees of franchisees. Previously, such companies were considered by law to be a step removed from many of their workers when certain labor disputes arose.

The decision, which came in a 3-2 vote on a single case before the board involving sanitation workers, is the latest to attempt to tackle the core question of who counts as an employee in a modern economy that is increasingly reliant on shift work, contract workers and other temporary employees.

The Labor Department, for example, has been cracking down on companies it says misclassify employees as independent contractors to avoid paying taxes, overtime pay and benefits. Uber Technologies Inc. is facing such accusations by drivers in California that the company has disputed.

Meanwhile, the NLRB has pending cases that claim franchisers such as McDonald’s Corp. should be held responsible for alleged labor law violations at independently owned restaurants.