A 22-year-old French citizen of Turkish descent, identified by authorities only as "A.S.", was arrested in Istanbul over suspicions of helping to plan a New Year's Day shooting at an Istanbul nightclub that left 39 people dead.
According to a Turkish police official, he was detained weeks ago and formally charged last week. His detention had not previously been made public.
The suspect was caught carrying the rental contract of the Istanbul house where the nightclub attacker, Uzbek national Abdulgadir Masharipov, was staying when he was captured on January 16.
On Saturday, a court remanded Masharipov, who has confessed to carrying out the shooting.
The Daesh terrorist organisation claimed responsibility for the attack, saying it was in revenge for Turkey's operations against the group in Syria.
Turkey, a NATO member, is part of the US-led coalition against Daesh. Its armed forces launched cross-border operations into Syria in August to drive security threats away from its borders.