By Stefan J. Bos
Sanchez was in as prime minister by Spain's King Felipe.
In a brief ceremony at the royal residence in Madrid, the 46-year old promised to "faithfully fulfill" his duties "with conscience and honor, with loyalty to the king, and to guard and have guarded the constitution as a fundamental state rule".
Sanchez, who is an atheist, took the oath without a bible or crucifix. That's a first in Spain's modern history.
Saturday's ceremony came a day after he ousted conservative Mariano Rajoy, who was one of Europe's longest-serving government leaders.
Sanchez plans to see out the remaining two years of the parliamentary term. But that won't be easy.
He had been barely sworn before one of the country's most critical issues facing his fragile government was pressed upon him: ending the Catalan secession crisis.
Not even two hours after Sanchez had taken his oath, Catalan chief Quim Torra demanded to meet with him and speak "government to government" about the future of the restive northeastern region.
Sanchez will also have to restore public trust in the government. Rajoy lost a historic no-confidence vote after corruption convictions last week involving former members of his conservative Popular Party.
As Spain's new prime minister, whose party only has a quarter of the seats in parliament, Sanchez now has to decide who to include in his cabinet.
He was expected to name them next week.