Aristide Gomes became Guinea-Bissaus prime minister on Monday in what President Jose Mario Vaz said would "put a definitive end" to years of political crisis in the small West African nation.
The ceremony came just hours after Vaz nominated Gomes for the post.
The president had told fellow leaders of the West African bloc ECOWAS on Saturday after consultations with political actors and civil society that Gomes, who served as PM of the former Portuguese colony from 2005 to 2007, would be a "prime minister of consensus".
Gomes is tasked with leading Guinea-Bissau to fresh parliamentary elections set for November.
Guinea-Bissau has been in the grip of a power struggle since August 2015, when Vaz sacked his then prime minister Domingos Simoes Pereira.
Vaz has since nominated several prime ministers but he has failed to gain the support of political parties.
Gomes, 63, succeeds Augusto Antonio Artur Da Silva, who was named in late January.