Bernard Cazeneuve has taken over as France's Prime Minister after Manuel Valls stepped down to focus on running for president in next year's election.
Mr Valls resigned a day after announcing his candidacy in the wake of President Francois Hollande deciding last week not to run for a second term.
Mr Valls hopes to unite Socialists and give the left a chance to stay at the Elysee Palace despite current opinion polls suggesting that the second round of the election could pit Marine Le Pen, the leader of the far-right National Front, against conservative Francois Fillon.
Mr Cazeneuve, 53, is a close ally of Mr Hollande and became a popular figure in French politics as the champion of measures tackling extremism in his interior minister role.
Mr Cazeneuve was appointed interior minister in 2014 and had to supervise the response to a series of attacks which have claimed more than 200 lives since January 2015.
In total, he championed three counter-terrorism laws and one intelligence law. He has also been in charge of implementing France's state of emergency following the Paris attacks that killed 130 people.
After a brief meeting with Mr Valls at the Elysee, Mr Hollande posted a brief message on Twitter announcing Mr Cazeneuve's appointment, which was also confirmed by the presidential palace press office.
Later, Mr Hollande said he had entrusted Mr Cazeneuve with protecting the French people.
"But protecting is not enough, we must prepare the future," Mr Hollande said in the Paris suburb of Villepinte.
A leading yet divisive figure of the Socialist party, Mr Valls is known for his outspoken, authoritarian style and his tough views on immigration and security. He will face other contenders in the Socialist primary next month before France's two-part presidential election in April and May.
Mr Valls has been harshly criticised by members of his own party after championing tough labour reforms and endorsing a controversial ban last summer on the Islamic "burkini" swimsuit.