Mr Sonko is President Faye’s political mentor.
The newly inaugurated president of Senegal, Bassirou Diomaye Faye, has named the leader of the biggest opposition party as his prime minister.
Ousmane Sonko is President Faye’s political mentor and the two had been imprisoned just before the March elections.
They were granted amnesty, alongside many other political prisoners, by the outgoing President Macky Sall.
“A few hours after taking office as the president of Senegal, his excellency Faye has appointed Sonko prime minister,” the presidency posted on X.
Mr Sonko had originally been named as the opposition’s presidential candidate until he was arrested and ruled ineligible to stand – a move that sparked mass protests.
During campaigns both men outlined radical political and economic reforms, including national reconciliation after months of turmoil sparked by their jailing.
In his inauguration speech on Tuesday, President Faye said Senegal would become a country of hope with a strengthened democracy.