ZeniMax employees at four locations in Maryland and Texas began voting through an online platform on Friday. The company owns major game franchises including The Elder Scrolls and Fallout.
Voluntarily agreeing to bargain with the union would allow Microsoft to avoid a formal election overseen by the U.S. National Labor Relations Board and the legal battles that often ensue.
CWA President Christopher Shelton applauded Microsoft's move in a statement, saying "when workers have the opportunity to join a union without company interference, it empowers them to have their voices heard in the workplace."
Microsoft did not immediately respond to a request for comment.
The company in June entered into an agreement with the CWA to remain neutral in union organizing campaigns at Activision Blizzard Inc, which Microsoft is seeking to purchase for $69 billion. The proposed acquisition is facing antitrust scrutiny from U.S. and European regulators.
Game testers at Activision units Blizzard Albany and Raven Software have voted this year to join unions amid claims by the CWA that the company has threatened and retaliated against union supporters. Activision has denied wrongdoing.
(Reporting by Daniel Wiessner in Albany, New York; Editing by Marguerita Choy)
By Daniel Wiessner