National Labor Relations Board tallied Raven Software workers' unionisation votes on May 23.
19 voted to unionise, while 3 voted against.
First time a major video game firm has a union.
Based on Activision's response to Raven's union attempts so far, discussions won't be quick or simple, Bloomberg's Jason Schreier tweeted.
Activision's first reaction to the layoffs that spurred the unionisation demonstration was unpopular with Game Workers Alliance members and sympathisers.