The tribunal ruled for former correspondent Richard Gizbert, who claimed ABC News ended his freelance contract last year because he would not cover the war in Iraq, where foreigners, including journalists, have been targeted for kidnapping and murder.
The American network says that all assignments to war zones and other dangerous areas are voluntary. It said it planned to appeal.
Gizbert, 48, is seeking $4 million in compensation, which the tribunal plans to rule on early next year.
The court rejected ABC News' contention that its dismissal of Gizbert was not linked to his refusal to cover wars, but came because he was inessential and it was making severe cutbacks.
It said testimony by ABC News executives was at times inconsistent and "not entirely reliable."
The tribunal said in a written verdict that "the principal reason for dismissing (Gizbert), in circumstances where (ABC News) was cutting back its budget, was his refusal to go to war zones."
"This ruling amounts to a vindication for an individual, it amounts to an indictment for one particular company, ABC News/Disney, and it's a warning to other news organizations that your voluntary war zone policy has to mean what it says," Gizbert said.
ABC News spokesman Jeffrey Schneider said the network would "vigorously appeal."
"This is a matter of principle for us," he said. "Mr. Gizbert was not dismissed because he wouldn't go to a war zone."
"It has always been our long-standing policy that these assignments are voluntary and that is understood by all the people who we work with," Schneider said.
Schneider defended the two ABC News executives whose testimony the tribunal criticized. "They are journalists of the highest integrity," he said.
Gizbert, who began working in ABC's London bureau in 1993, had been a war correspondent for years, covering conflicts in Bosnia, Chechnya and Somalia for ABC. He said he became reluctant to do so in the late 1990s, as his children grew up.
ABC initially had little problem with his decision, said Gizbert, who is from Ottawa. But after the 2001 attacks on America, an intense new focus on foreign news meant London correspondents were badly needed in war zones.
Gizbert said he gave up at least $150,000 a year in salary and benefits to go freelance so he wouldn't have to cover conflicts.
ABC News decided last year not to renew his contract, a decision the tribunal said amounted to dismissal.
Gizbert's lawyer Patrick Green said the ruling "shows that the media do not get to make their own rules."
"It marks an important step in the case law, by recognizing the global workplace and the real risks of reporting from war zones," he said.