The union is supporting Mark Lyon, who is claiming unfair dismissal after he was sacked by Ineos, which owns the site in Scotland.
The tribunal, sitting in Edinburgh, ruled it was "likely" that Mr Lyon will win his case for unfair dismissal when it goes to a full tribunal and ordered Ineos to pay his wages until the full hearing in a number of months, said Unite.
The convenor, who had worked at Grangemouth for 25 years, was dismissed for not stopping the union commenting on media reports about fears of job losses at the plant, Unite maintains.
Unite's legal director Howard Beckett said: "We welcome this interim finding which gives Mr Lyon some financial security until the full tribunal where all the evidence will be heard.
"It is a shot in the arm for workers across the country and sends out a clear message that they can be a member of trade union and represent other workers without fear of victimisation.
"Ineos needs to drop its hostility to the workforce and ensure there is no victimisation of workplace representatives before the brain drain of skills at the site becomes a flood that threatens the site's survival.
"Ineos should be in no doubt that we will continue to fight for our members at Grangemouth and pay heed to the interim finding by starting to work with the representatives the workforce has chosen."
Neil Findlay of Scottish Labour, said: "I wholeheartedly applaud today's ruling, which is not only good news for workers and their rights at Ineos, but also for ordinary working people across the country.
"The court has put Ineos on notice that illegal harassment and intimidation against employees and their trade union representatives should end.
"This ruling sends a message to all employers that anti-worker, anti-trade union behaviour will not be tolerated.
"I continue to stand with Unite, its members at Ineos and with all working people in this country as they push for fair wages and treatment on the job.
"To ensure a strong economy Britain workers and trade unionists must be treated with respect, not bullied."
Grangemouth was at the centre of a dispute last year which almost led to the closure of part of the plant.
Workers had threatened to go on strike but later accepted changes to terms and conditions.