The company must now hold a meeting to advise workers of their rights, and CEO Elon Musk is required to attend.
Administrative Law Judge Amita Baman Tracy ruled Friday that Tesla had violated the National Labor Relations Act multiple times in 2017 and 2018.
The judge said the tweet amounted to “threatening employees” they would lose their stock options if they voted to unionize.
“Musk’s tweet can only be read by a reasonable employee to indicate that if the employees vote to unionize that they would give up stock options,” Tracy wrote. “Musk threatened to take away a benefit enjoyed by the employees consequently for voting to unionize.”
The ruling also found Tesla violated labor laws by prohibiting employees from distributing leaflets in their off-hours in the employee parking lot in Fremont, California; telling employees it was futile to vote for the union; and interrogating employees about their union activities.
Tracy ruled that Tesla needs to “cease and desist” that behavior and fully reinstate and compensate one employee who had been unlawfully fired for being involved with union activities. The carmaker also needs to inform workers at a meeting that Tesla violated the National Labor Relations Act several times.
The UAW on Saturday did not immediately comment on the ruling. It said Tesla workers have not yet held a vote on unionizing.
Tesla did not immediately comment Saturday. It is not clear if the company will appeal.
In 2017, Tesla said that no employee had ever been punished for supporting a union, in response to a complaint from UAW.
UAW said on Saturday its contracts with General Motors, Ford and Chrysler did have profit-sharing plans.
CNN Business’ Chris Isidore contributed to this report.