A key question will be whether Barnier's government will manage to get reforms adopted by a deeply divided parliament.
The discreet, conservative politician will quickly face a baptism of fire as time is running out to prepare France's 2025 budget, which could trigger a vote of no confidence if other parties are not satisfied.
At 73, Barnier will be the oldest prime minister in France's modern political history, taking over from Gabriel Attal, who was the youngest.
Barnier first became an MP at age 27 and later held roles in several French governments, including foreign minister and agriculture minister.
He is best known abroad for having led the EU's talks with Britain over its exit from the bloc from 2016 to 2021.
Macron had considered a string of potential prime ministers in recent weeks, none of whom mustered enough support to guarantee a stable government, which was his key criteria.
President Emmanuel Macron had considered a string of potential prime ministers in recent weeks. (AP PHOTO)
There is no guarantee Barnier's government will be able to convince parliament to adopt reforms or keep him in power for long.
But at least the far-right National Rally (RN), the biggest single party in parliament after the early July election, signalled earlier on Thursday it would not immediately reject Barnier if he met certain conditions.
Barnier is a staunch pro-European and a moderate career politician, but he toughened his discourse considerably during his failed 2021 bid to get his conservative party's ticket for the presidential election, saying immigration was out of control - a view shared by the RN.
Barnier's political views are overall quite close to Macron's, and it was crucial for the French president that his new prime minister would not try to undo reforms pushed through in the past years, in particular Macron's pension reform.
He will need in any case to negotiate with other parties to get legislation adopted in parliament.
As Macron's hunt for a prime minister has dragged on, public finances have deteriorated and outgoing Finance Minister Bruno Le Maire says tens of billions of euros in budget cuts are needed to plug the hole.
Macron's gamble to call the snap parliamentary election in June backfired, with his centrist coalition losing dozens of seats and no party winning an absolute majority.
Voters did, however, rally to deny the RN a victory.
The left's New Popular Front alliance came first but Macron ruled out asking them to form a government after other parties said they would immediately vote it down. Instead, he waited weeks to make his choice.
The left on their own will not have enough votes to vote Barnier down, but could call street protests.
RN MP Sebastien Chenu told BFM TV the far-right party would wait to see what Barnier has to say on immigration, and on changing France's voting system.
MP Laurent Jacobelli, also from the RN, said a condition for its support was that parliament be dissolved as soon as possible, which would be early July.
Still, the RN was not particularly enthusiastic about Barnier.
"They are taking out of mothballs those who have governed France for 40 years," Jacobelli told TF1.
