Last week, the Constitutional Council had confirmed his victory with 61 per cent of the vote in the presidential election of May 6.
Déby had taken power with a group of generals in 2021 after the death of his father, Idriss Déby Itno, and suspended the constitution.
His father had previously ruled the country for more than 30 years.
Since 2020, seven countries in Africa have experienced successful military coups, almost all of them in Francophone West and Central Africa.
Chad is the first country in the so-called Coup Belt in the Sahel region to hold elections since then.
Déby is the sixth president in the history of the country of around 19 million inhabitants, which has never experienced a peaceful transfer of power since its independence from the former colonial power France in 1960.
The Constitutional Council rejected objections to the election result by the opposition, which, like representatives of civil society, spoke of massive electoral fraud.
On Wednesday, runner-up Succès Masra resigned from the post of prime minister, which he had held in the transitional government since the beginning of the year.
On Thursday, Déby appointed diplomat Allah-Maye Halina as the new head of government, state television reported.
The appointment of the confidant, who had served as head of protocol under Déby’s father and was most recently ambassador to China but has no government experience, is considered a surprise.
Unlike other military rulers in the region, Déby is an important ally of France, which has redeployed its troops to Chad following coups in the Sahel region.
Chad is located at an important crossroads between the crisis states of Sudan, Libya, Niger, and the Central African Republic and is home to more than one million refugees.
