But Mr Sanchez's appointment was quickly disrupted by Catalan's newly elected leader Quim Torra, who demanded the newbie meet him to negotiate wealthy Catalonia becoming an independent state.
Unlike the new populist government in Italy, Sanchez and his party are staunch supporters of the European Union and its shared euro currency.
As Spain's new leader, whose party only has a quarter of the seats in Parliament, Sanchez now has to decide who to include in his cabinet and is expected to name them next week, the report said.
"Mr Sanchez is concerned because he is not doing any better in the polls, and he has realised that he will never reach government through elections", he said.
Rajoy, 63, must offer King Felipe VI his resignation, according to media reports, before his successor can take office.
The 46-year-old's Socialist party holds just 84 seats in the 350-member assembly, making it unclear how long his administration can last.
A Basque nationalist party's announcement that it would vote in favor of the motion spelled the nearly certain end of Prime Minister Mariano Rajoy's mandate and foretold the stunning collapse of his minority government in a parliamentary vote Friday, when it will be short of support to survive.
But Anton Losada, a politics professor at the University of Santiago de Compostela who has written a book about Rajoy, said the party may not be completely over.
'Just an overreaction' for officers to repeatedly punch woman: Defense attorney
It should be noted that the breathalyzer came out negative after Weinman was instructed to take the test by officers. Images from the video released earlier this week shows an officer punching a woman on a beach in New Jersey.
Trained as a lawyer, Rajoy turned to politics at a young age, joining the Popular Alliance, the party founded by ministers of former dictator Francisco Franco which later became the PP.
He has also vowed to implement the 2018 budget drawn up by Rajoy's government.
He earned a stunning victory to return as leader in May past year when he won an internal party election against Susana Diaz, the candidate anointed by the party's powerbrokers, including former prime ministers Felipe Gonzalez and Jose Luis Rodriguez Zapatero.
While Rajoy looks likely to be unseated by the Socialist party which presented the no-confidence motion, the veteran politician isn't going down without a fight.
"Tell us, why are four courts investigating the Valencia socialists?"
He backed Rajoy's imposition of direct rule on the region after separatist leaders declared Catalan independence.
After Mr Sanchez won the support of Basque and Catalan nationalist parties during the parliamentary vote, Mr Rivera said his party would be "very attentive to the concessions" that are made. In 2003 he took a position as a city council member in Madrid and then climbed up the Socialist party ladder, first becoming an MP and then party leader in 2014. It will face a political minefield as it tries to steer legislation through parliament by winning support from rival parties.