The judge says the firm committed “egregious and widespread” violations of federal labour law at its stores in cities of Buffalo and Rochester.
The coffee chain has been ordered to rehire the ex-workers and compensate others who were affected.
In response, Starbucks said its actions were lawful and in line with policies.
“We believe the decision and the remedies ordered are inappropriate given the record in this matter and are considering all options to obtain further legal review,” Starbucks told the BBC.
In a more than 200 page decision, Michael A Rosas, who is a federal administrative law judge, also ordered Starbucks to rehire another worker that he ruled had been illegally forced out of their job.
Mr Rosas said Starbucks had showed “a general disregard for the employees’ fundamental rights” in response to union campaigns.
The judge added that the company interrogated and threatened workers and restricted discussion of pay.
It also repeatedly sent high-ranking staff to stores “in order to more closely supervise, monitor, or create the impression that employees’ union activities are under surveillance,” he said.
The NLRB is an independent federal agency with the power to safeguard employees’ rights to unionise at their workplaces.
Michael Sanabria, a Starbucks barista in Buffalo, said the ruling was “such a massive win for us, and for the labour movement as a whole.”
“The decision results from months of tireless organising by workers… demanding better working conditions in the face of historical, monumental, and now deemed illegal union-busting,” said Michelle Eisen, a barista at another Starbucks location in Buffalo.
Last year, workers at around 270 of Starbucks stores across the US voted to join unions, despite opposition from the firm. Starbucks has around 9,000 company-owned stores in the US.
Union campaigners have accused the company of dragging its feet at the bargaining table and violating labour laws as it tries to shut down the movement.
Starbucks – which has long prided itself on its reputation as a progressive, worker-friendly employer – has said it respects workers’ right to protest.
In recent months the company has raised pay and made other changes in response to the discontent.