Police sources say the man, whose name has not been made public, was arrested near the north-eastern French city of Lille following Tuesday's arrest in Brussels of Akim and Khalid Saouti, who have been charged with membership of a terrorist group.
The French suspect had been jailed four times for acts of violence and theft and was placed under surveillance on his release from prison in May 2015 because he was believed to have been radicalised while in detention.
He was observed making several trips to Belgium, where he met the Saouti brothers.
All three were seen handling bags in a garage in the Brussels district of Anderlecht.
Belgian police feared an imminent attack and launched a "wave of arrests", sources told the AFP news agency.
Firearms and police uniforms found
A raid on the garage revealed three Kalashnikov assault rifles, a pump action shotgun, three handguns, four detonators and ammunition, as well as the uniforms.
One of the bags was found at the French suspect's home.
It contained several kilos of jewels, according to the source, who said investigators want to know whether he had emptied it or had provided weapons to the Saoutis.
Jihadist propaganda was found on his computer.
Saouti brothers' criminal record
The Saouti brothers were already well-known to the police.
Khalid, 37, helped another brother, Mohamed, escape from prison in 2002.
The pair were eventually caught and Mohamed returned to jail, while Khalid was given a one-year suspended sentence.
Another brother, Saïd, is a former leader of a motorcycle gang known as the "Kamikaze Riders", which was founded in 2003.
He was found guilty of membership of a terrorist organisation last year, although charges of preparing a terror attack were dropped for lack of material proof.
Investigators from a joint French-Belgian operation on Wednesday confirmed the arrest of a 42-year-old man with alleged links to the Kamikaze Riders near Lille on suspicion of planning a violent attack.