Unite said it will take legal action on behalf of Mark Lyon, who was said to have been dismissed for not stopping the union from commenting over fears of job losses at the plant.
The move follows the resignation of Unite's other Grangemouth convenor, Stevie Deans, last year following a long-running row over his involvement in the selection of a Labour candidate in Falkirk.
Unite said there was significant medical evidence that Mr Lyon was suffering from a serious stress-related illness as a result of the treatment he had "endured" at the hands of Grangemouth's owners Ineos.
The union said: "Unite believes that Ineos was determined to rush through a disciplinary process against Mr Lyon, denying his legal representatives the appropriate time to prepare his defence.
"Unite will be appealing the company's decision and issuing an employment tribunal claim for unfair dismissal and victimisation for trade union reasons."
Pat Rafferty, Unite's Scottish secretary, said: "Once again, a decent man and loyal employee has been hounded out of his job by Ineos.
"The rank hypocrisy of Ineos knows no bounds. The company withdrew its defamation action against Unite to 'draw a line under the dispute', yet at the same time it was drafting a letter dismissing a Unite convenor.
"Mark Lyon has been subjected to a grotesque mockery of the disciplinary system which saw him tried in his absence. While he attended a doctor's appointment, his employer was sacking him on trumped up charges. From start to finish, this has been a circus and reflects extremely poorly on Ineos.
"Ineos has run away from the defamation action and Unite stands by its assertion that Stevie Deans was victimised. Ineos has no interest in drawing a line and moving forward, just confrontation and conflict.
"We would urge Ineos to think again about a decision that is bound to send shock waves through the workforce. Worryingly skilled people are choosing to leave Grangemouth. Ineos needs to realise that this is not a time for further unrest and that they need to get the workforce on side to deliver a successful future."
Unite said the sacking would send "shockwaves" through the workforce, and warned that skilled workers are already quitting their jobs.
Grangemouth was at the centre of a dispute last year, which led owners Ineos to threaten to close part of the site if workers didn't agree to a rescue package to help secure its future.
Unite had been threatening strikes over the suspension of Stevie Deans, but the union decided to accept the company's proposals, and Mr Deans resigned from his job.
He was also chairman of the Falkirk Labour Party, which was embroiled in a dispute over claims that Unite had tried to influence the selection of a candidate.
An internal report was drawn up on the saga for the party, with Unite insisting it had done nothing wrong.
The row led to Labour leader Ed Miliband announcing reforms in the historic link between the party and unions - changes which were agreed today by the party's national executive.
A company spokesman said: "Ineos has no comment to make at this time. This is a confidential issue between the company and Mr Lyon."