Adlene Hicheur, a former researcher at Switzerland's CERN laboratory, was convicted of "criminal association with a view to plotting terrorist attacks."
Hicheur, who has been behind bars since he was arrested in October 2009, could have received up to 10 years in prison.
The 35-year-old scientist and his defenders say he was a victim of allegedly overzealous French anti-terrorism laws and that he explored ideas on jihadist websites but never took any concrete step toward terrorism.
Speaking after the judgment, Hicheur's lawyer called the verdict "scandalous."
Lawyer Patrick Baudouin said Hicheur hasn't decided whether to appeal the verdict. If he does not, with time off for good behavior, his client "should be out rather quickly," he added.
The case centered on about 35 emails between Hicheur and an alleged contact with Al-Qaida in the Islamic Maghreb named Mustapha Debchi, who tried to convince him to carry out a suicide bombing. Hicheur declined, but in one response suggested striking at the barracks of a battalion of elite Alpine troops in the eastern town of Cran-Gevrier.
Hicheur claimed he was on morphine for a herniated disk and going through a personal "zone of turbulence" when he wrote an 2009 email that advocated an attack on the barracks.
Prosecutor Guillaume Portenseigne rejected Hicheur's claims of a lack of lucidity and characterized the defendant as "a man who had everything going for him ... but just got led astray in a radical jihadist Islam."
At the two-day trial in March, the prosecutor called Hicheur "a budding terrorist" who only needed a "determining meeting" to slip into concrete action.
Hicheur's lawyer argued that "everything has been done to demonize" his client, "to make him into ... France's most dangerous terrorist, potentially susceptible to participate in a bombing."
Hicheur's defenders said recent terror attacks in France did not help his case.
In an apparently unrelated case in March, police say another young man of Algerian descent killed three Jewish schoolchildren, a rabbi and three paratroopers in the cities of Toulouse and Montauban and claimed ties to al-Qaida. Mohamed Merah, 23, died later in a shootout with police.