After deliberations with Romania's political parties, President Klaus Iohannis nominated Ciolacu of the leftist Social Democratic Party (PSD) to try to form a government.
Ciolacu, whose party topped the polls in a December 1 parliamentary election, has served as Prime Minister since June last year.
It is widely seen as a tactical push to shut out far-right nationalists who made significant gains in parliament, revealing widespread anti-establishment sentiment.
"It will not be an easy mandate for the future government," Ciolacu said.
"We are aware that we are in the midst of a deep political crisis. It is also a crisis of trust, and this coalition aims to regain the trust of citizens, the trust of the people."
The president's nomination will need to be approved by legislators.
The parliamentary election came on the heels of a presidential vote in which the far-right outsider Calin Georgescu won the first round, in which Mr Ciolacu came third.
Georgescu's surprise success plunged Romania into turmoil as allegations of electoral violations and Russian interference emerged.
Days before the December 8 presidential run-off, the Constitutional Court made the unprecedented move to annul the presidential race.
The new coalition is expected to comprise the PSD, the centre-right National Liberal Party (PNL), the small ethnic Hungarian UDMR party and national minorities.
The three parties have also agreed on a common pro-European candidate to enter the rerun of the presidential election, the date of which is yet to be announced.
"Each of the signatories of this political agreement has understood the signal sent by Romanians during the general elections," Ciolacu said.
In 2021, the PSD and the PNL also formed an unlikely but increasingly strained coalition together with UDMR, which exited the cabinet in 2023 after a power-sharing dispute.
