The man, identified as Endri E, is a 28-year-old Albanian suspected of having supplied a gun to Mohamed Lahouaiej Bouhlel, the Tunisian who carried out the attack on 14 July 2016, France's Bastille Day national holiday.
Police arrested the suspect at Sparanise, near the southern city of Naples, acting on a European arrest warrant issued by France.
The arrest comes after the Paris appeal court in March ordered eight people to stand trial for their alleged role in the 2016 attack.
That trial is scheduled to open in 2022.
Perpetrator shot dead at the wheel of attack vehicle
Bouhlel himself was shot dead by police at the wheel of the vehicle he had used to kill people as they celebrated the holiday on Nice's waterfront. More than 400 people were injured.
The Nice attack came less than a year after the Bataclan assault in November 2015 in which 130 people lost their lives in bombings and shootings across the French capital.
The Islamic State terrorist organisation claimed responsibility for the Nice attack. Investigators have found no evidence that Bouhlel had sworn allegiance to the jihadist group.
