Posting a video on YouTube, the group said it would use its knowledge to "unite humanity" and warned the terrorists to "expect us".
Behind their signature mask, a spokesperson said: "Anonymous from all over the world will hunt you down.
"You should know that we will find you and we will not let you go. "We will launch the biggest operation ever against you. "Expect massive cyber attacks. War is declared. Get prepared.
"The French people are stronger than you and will come out of this atrocity even stronger."It isn't the first time the hackers have made threats against ISIS as they made similar threats after the Charlie Hebdo attacks in January.
The 'hacktivists' run an operation publishing a list of Twitter accounts it claims are spreading propaganda in support of ISIS.