The French agency, which will be run by the Secretariat-General for National Defence and Security (SGDSN), will employ 60 people to trawl through online content, SGDSN chief Stephane Bouillon told parliament.
Bouillon told the National Assembly's defence committee that the initiative was "not about correcting or establishing the truth" but identifying attacks that come "from a foreign country or organisation that aims to destabilise the state politically".
The agency would help "politicians, diplomats, the judiciary and the press to realise that, out of the 400,000 tweets on this or that news item, 200,000 are coming from a bot farm in a foreign region or that a particular debate is coming from a troll farm," he added.
During the last French presidential election in 2017, groups linked to Russia were blamed for a massive hacking attack on the party of centrist President Emmanuel Macron, who faced anti-EU nationalist Marine Le Pen in the second-round runoff.
Russian hackers are also accused of having tried to tilt the 2016 US election in favour of former president Donald Trump and of meddling in Britain's Brexit vote.
Moscow denies hacking or meddling in foreign elections.
Not an intelligence service
Bouillon insisted that the new French agency, which will be launched in September, would not act as an intelligence service.
Its activities will be vetted by an ethics committee whose members will be drawn from the judiciary, diplomatic corps, the media, and the research community, he said.
"Our objective is to detect something that is spreading as quickly as possible and to flag up the arsonist."
"Once the forest has burned it's sad but it's too late," Bouillon said.
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He said that the SGDSN would be closely watching September's parliamentary election in Germany in the hope of learning lessons from it.
France will also be watching social media closely in December when the French Pacific territory of New Caledonia holds its third referendum on independence, to determine "whether there are countries who have an interest in seeing the result take a direction that is not necessarily the one that New Caledonians themselves would vote for," he said.
The neighbours already have one
France is not the first country to create a government agency to fight disinformation.
The US State Department's Global Engagement Center is tasked with identifying and countering foreign propaganda and disinformation.
In 2018, the British government also set up a unit to combat disinformation "by state actors and others".
Three years earlier, the EU established the East StratCom Task Force with the stated intention of fighting Russian anti-EU campaigns.
