Workers at the Brooklyn-based firm voted 46 to 37 to unionise in a first for major internet firms long known for providing generous pay and perks to employees.
"We are a union," Kickstarter United, which was a voice for the union effort, tweeted in a message on Tuesday.
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"And to all tech and creative workers looking to fight for your rights, this is only just the beginning."
Kickstarter workers will be affiliated with a New York City local of the Office and Professional Employees International Union (OPEIU).
"I congratulate the workers of Kickstarter, the first major tech company to vote for a union," tweeted Bernie Sanders, a self-described democratic socialist who is vying to be the Democratic Party contender in November's US presidential election.
"These workers are setting an example for the entire industry."
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The OPEIU last year adopted a resolution to organize tech sector workers, citing an "astronomical increase" in interest in unionising.
The union noted protests or petitions by employees at Amazon, Google, and video game studio Riot Games as signs that tech company workers are uniting to challenge decisions or conditions they find unacceptable in workplaces.
According to US government statistics, 10.3% of American workers belonged to unions last year, down from 20.1% in 1983.
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