Politicians and union leaders have welcomed a ruling which stops the US imposing huge 292% import tariffs on planes partly made by British workers.
Over a thousand jobs in Belfast depend on the success of the C-Series passenger jet, which is manufactured by Canadian aerospace firm Bombardier.
UK Prime Minister Theresa May said it was "good news" for UK industry.
Trade union Unite said workers in Northern Ireland "will be breathing a huge sigh of relief".
Bombardier had been widely expected to fail in its bid to overturn a ruling by the US Commerce Department in December that the UK and Canada had given it unfair subsidies.
But the case, centred on a complaint by US rival Boeing, was dismissed by the US International Trade Commission (ITC).
Mrs May welcomed the decision, saying: "Bombardier and its innovative workforce play a vital role in the Northern Ireland economy."
Gavin Robinson, Democratic Unionist Party MP for East Belfast, said it had been a "very difficult time" for the firm's 4,000 workers in Northern Ireland who had "faced an uncertain future".
He added: "Bombardier's greatest strength here in Belfast is the quality of those workers and the product they deliver."
Steve Turner, Unite assistant general secretary, said Bombardier workers "will be breathing a huge sigh of relief that the International Trade Commission has seen through Boeing's baseless complaint".
The ITC voted 4-0 in favour of Bombardier, ruling that there was no injury to US manufacturers.
Tariffs of 292% will not now be imposed on orders of C-Series planes by US airlines.
About 50 companies in the UK supply Bombardier with parts for the C-Series.
The wings for the plane are made by 1,000 workers in Belfast.
The BBC's business correspondent, Theo Leggett, said the move came as "a big surprise" as "most analysts thought the odds were stacked against Bombardier".
"It is good news for workers at Bombardier's Belfast plant, where parts for the C-Series are made, but it's also good news for Airbus, which took advantage of Bombardier's struggles to take a majority stake in the C-Series."
A spokesperson for Bombardier said the ITC's decision was "a victory for innovation, competition and the rule of law."
"Its development and production represent thousands of jobs in the United States, Canada, and the United Kingdom."
A spokesperson for Boeing said it was "disappointed" by the ITC's decision and that it would "review the detailed conclusions when they are released".
But Business Secretary Greg Clark said: "The decision by the International Trade Commission confirms what the UK and Canadian governments working hand in hand has maintained from the outset, that this case is unjustified. We are pleased that the ITC have now recognised this."
It was "excellent news for the dedicated workforce in Northern Ireland and supply chain across the UK, who have a great future ahead," he added.
Sinn Féin's northern leader Michelle O'Neill said the ruling was a "welcome victory" for the Belfast workforce and she hoped "the matter was now closed".