The Unite 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" 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 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 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.This ruling sends a message to all employers that anti-worker, anti-trade union behaviour will not be tolerated.
Ineos said in a statement: "These rulings do not change Petroineos' view that Mr Lyon was guilty of repeated misconduct and that his dismissal was entirely justified, subject to appeal."