Mr Bunel says he was acting under orders
Pierre-Henri Bunel, 49, was jailed for two years, with a further three years suspended.
He was immediately taken to the Sante prison in Paris.
A special military court in Paris heard that he revealed details of Nato's bombing plans just before its military campaign got under way in Kosovo.
Bunel, who was attached to Nato in Brussels at the time, admitted passing on information, but denied the treason charges, saying he was acting under the orders of French intelligence services.
"I committed a serious mistake but I do not have the feeling that it was treason," he said after the verdict.
Bunel had claimed he was told to convince the Serbs that the threat of Nato bombardment was real, unless they withdrew their troops from Kosovo.
He had received a phone call via "a line that could only have been a military line", he said, telling him he should convince Jovan Milanovic, a Serbian agent based in Brussels, that Nato was serious.
"I want to show that I am not a traitor. I want to do this for my honour, for my family, which has suffered a lot as a result of this affair, and also for my fellow soldiers, who do not understand how I could have committed treason," he said in comments broadcast on French radio before the verdict.
Nato strains
The potential Nato targets allegedly identified by Mr Bunel were, in the event, not hit as the alliance postponed the threatened strikes.
Other alliance members, however, have blamed Serbian sympathies within the French military for hampering Nato's campaign in the Balkans.
Prosecutors had asked for a five-year term for Bunel.
"You wanted to be a hero but you were a traitor. You must assume the consequences," state prosecutor Janine Stern told the tribunal.
"You betrayed your comrades, you betrayed your allies, you betrayed France."