"This election marks an essential stage in stabilisation and political and economic recovery of Mali, which still faces the challenges of terrorism, insecurity and development, whose seriousness at present makes national unity and reconciliation urgent," Le Drian said in a statement.
Despite objections from opposition candidate Soumaïla Cissé, Mali Constitutional Court on Monday declared Keïta the winner of the election, whose second round took place on 12 August.
During his second term, which begins on 12 September, he hopes to salvage the peace agreement reached with former Tuareg rebels in 2015, and reduce the Islamist violence that still affects the north and centre of the country, as well as spreading to neighbouring Niger and Burkina Faso.
"Malians have especially shown that they will not be intimidated by terrorist acts," Le Drian commented.
One polling official was killed in the northern city of Timbuktu during the election, but the number of polling stations that could not open was halved in the second round compared to the first round, reducing to 490 out of 23,000.
President Emmanuel Macron congratulated Keïta on his reelection when provisional results were announced on 16 August.
Hostage's family accuse Macron
A nephew of a French woman held hostage in Mali since late 2016 has called on Macron to meet her family, complaining that nothing is being done to obtain her release.
Sophie Pétronin, 75, is suffering from malaria and cancer, according to her family.
"You can't let somebody die like that," Arnaud Granouillace told a regional TV bulletin.
"We know perfectly well that M Macron does not want to negotiate but he could at least have the decency to tell us to our faces," he added.
The family is to meet Nouvelle-Aquitaine regional council chairman Alain Rousset on 29 August and a banner with Pétronin's portrait on it is displayed on the council's seat in Bordeaux.
In July Macron said the government was working "tirelessly" for her release.
Pétronin, who was running an NGO for orphans, was kidnapped in Gao, northern Mali, on 24 December 2016.
In 2017 a jihadist group linked to al-Qaeda distributed a video of six foreign hostages, including her.
Another video distributed in June, in which she addressed her son, Sébastien, was "proof of life", according to the family.