The authorities said the suspects detained Monday were French and Tunisian and had links to the attacker, Mohamed Lahouaiej B., who plowed a 19-tonne truck into a large crowd enjoying a July 14 fireworks display on Nice's famed seafront Promenade des Anglais.
All eight were arrested in the Alpes-Maritimes region in the southeastern corner of France that includes Nice.
It was not immediately clear what role the latest suspects played in the massacre.
At least five people already face preliminary terrorism charges in the attack, and are accused of helping the attacker obtain a pistol and providing other support.
The so-called Islamic State (IS) group has claimed responsibility for the July 14 attack.
French authorities say the attacker, a Tunisian with French residency -- who was shot dead by police during the attack -- was inspired by the extremist group's propaganda.
The IS group said in a statement that Mohamed Lahouaiej B. was a “soldier” and that he was following their call to target citizens of countries fighting the extremists.
However, the police have yet to find any evidence of a concrete link between Mohamed Lahouaiej B. and the terrorist network.
The Nice attack was France’s third major terror attack in 18 months following the January 7-9, 2015 “Charlie Hebdo” attacks and the November 13 Paris attacks, which killed 130 people.
A state of emergency remains in place in France since last November’s attack.