Paul Chambers was found guilty of sending a menacing electronic communication last year (10) for writing a message on the microblogging site threatening to blow Robin Hood Airport in Doncaster, northern England "sky high" after snowy conditions closed the building and he missed a flight.
Fry, famous for his love of Twitter, was outraged by the conviction - especially as Chambers insisted the post was meant as a joke - and the comedian even offered to pay his legal costs and fine.
Chambers had an initial appeal turned down in November (10) but he is continuing to fight his case, and Fry helped organise a fundraising show in London to bankroll his legal battle.
Fry took to the stage at the city's Bloomsbury Theatre and made a passionate appeal on Chambers' behalf, telling the audience, "This (verdict) must not be allowed to stand in law."
Chambers' lawyer, David Allen Green, added, "(The guilty verdict) does not make me proud to be an officer of the court. We should be able to have banter. We should be able to speak freely without the threat of legal coercion."
Chambers was found guilty in May 2010 and ordered to pay a $616 (£385) fine and costs. His appeal is likely to go before Britain's High Court later this year (11).