Alexis Tsipras has received the formal mandate to form a government for the second time this year, after his left-wing Syriza party won Greece's elections with an unexpectedly wide margin.
Syriza's victory marks a personal triumph for Mr Tsipras, who served as prime minister between January and August - a tumultuous period that saw Greece's future in the 19-country Eurozone come under real threat and strict banking controls imposed.
At 41, Mr Tsipras dominates Greek politics despite a major policy U-turn that saw him go against the anti-austerity platform that swept him to power in elections in January in return for a multi-billion bailout that keeps Greece in the Eurozone.
Mr Tsipras, who has seen off both the main, centre-right opposition and his own party rebels, has said he will renew his pro-European coalition with the small, right-wing populist Independent Greeks (ANEL) party, which beat opinion polls to clear the three percent threshold required for representation in parliament.
The new government will have a small majority of just five seats, but, according to Mr Tsipras, is aiming to see out a four-year mandate.
"We now have the great opportunity, taking steady steps and using the four years of our mandate, to implement our main commitment, which is to give an honest fight and to shed our blood if necessary to stop our people bleeding further," he said after a meeting with ANEL leader Panos Kammenos.
Mr Kammenos himself promised a "progressive" administration that would build on the previous coalition's work, adding that the cabinet would be announced by Wednesday morning at the latest.
Congratulating Mr Tsipras, European Union president Donald Tusk said he hoped the election results "will now provide for the political stability necessary to face all the challenges at hand".
As well as noting Greece's own financial difficulties, Mr Tusk highlighted the immigration crisis that Europe is struggling to deal with. Greece has registered 260,000 refugees and economic migrants this year alone.
And the German government "will also work closely and in partnership with the new Greek government," spokesman Steffen Seibert told reporters in Berlin.
German chancellor Angela Merkel spoke with Mr Tsipras by phone to congratulate him, and discussed the refugee issue Europe currently faces as well as other issues, Greek and German officials said.
European Commission president Jean-Claude Juncker also called Mr Tsipras to congratulate him, Greek officials said.
