Three masked attackers opened fire with assault rifles in the office and exchanged shots with police in the street outside before escaping by car.
The gunmen shouted "we have avenged the Prophet Muhammad", witnesses say.
President Francois Hollande said there was no doubt it had been a terrorist attack "of exceptional barbarity".
A major police operation is under way in the Paris area to catch the killers. The number of gunmen was initially reported to be two, but French Interior Minister Bernard Cazeneuve later said they were hunting three "criminals".
Four of the magazine's well-known cartoonists, including its editor-in-chief Stephane Charbonnier were reported among those killed, as well as two police officers.
Mr Charbonnier, 47, had received death threats in the past and was living under police protection.
French media have named the three other cartoonists killed in the attack as Cabu, Tignous and Wolinski. Reports say the attack took place during the magazine's daily editorial meeting.
Several of the wounded remain in a critical condition.
The satirical weekly has courted controversy in the past with its irreverent take on news and current affairs. It was fire-bombed in November 2011 a day after it carried a caricature of the Prophet Muhammad.
12 killed by gunmen in France over Mohammed cartoon
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Just another set of idiots who take things out of context. I agree with Inzane, those gunmen should be found and have their nuts cut off and left to bleed to death. Fucking idiots.
You can't paint all muslims with the same brush as a few (well more than a few) retards.
not a single option missing including infrared night driving
