The 62-year-old mayor of the western city of Nantes, is a long-term ally of the new president. The two men have known each other for 15 years and are said to share a mutual respect.
Hollande is said to appreciate Ayrault's "discreet and measured" temperament and his "ordinary" qualities - a word used to describe Hollande himself more than once during the presidential campaign.
Like Hollande, Ayrault has no direct ministerial experience.
At the age of 26 he became the country's youngest-ever regional councillor. In the following years he gradually made his way through the party ranks and was elected minister of the Loire Atlantique region in 1986 and then mayor of Nantes in 1989.
In 1997, while Lionel Jospin was prime minister, he was chosen as the head of the Socialist parliamentary bloc in the National Assembly where he has remained ever since.
The only blot on his copy book is a suspended sentence and 4,500-euro fine received in 1997 for an improperly tendered contract to print a Nantes local newspaper.
During the election campaign, the media seized on the incident, saying it undermined Hollande's vow to clean up politics.
Ayrault defended himself saying he took responsibility for the mistake as he was mayor at the time but he was not personally involved in the affair.
Ayrault's training as a German teacher may have been a point in his favour, given the expected difficult negotiations the new president will hold with German Chancellor Angela Merkel.
He travelled to Berlin last year on Hollande's behalf to meet a senior Merkel adviser in a bid to build relations with her conservative government.
In his down time, he is said to be an excellant tango dancer.