Yingluck, a 44-year-old political novice, was elected as Thailand’s first ever female prime minister in a parliamentary vote on Friday but had to complete formalities including official approval by King Bhumibol Adulyadej.
“His Majesty the King has given his endorsement,” House Speaker Somsak Kietsuranont told AFP after receiving an audience with the 83-year-old monarch, who has been in hospital since September 2009.
Yingluck swept to an election victory last month with the support of her older brother Thaksin, who is loathed by the elite in government, military and palace circles, and was toppled in a 2006 coup.
Bhumibol, who has reigned for more than six decades, is seen as a stabilising force in the troubled country but has no official political role.
The businesswoman, described by her brother as his “clone”, faces the tough challenge of bringing unity to the politically volatile kingdom.
Thaksin, a former telecoms tycoon, lives in self-imposed exile to avoid a jail sentence imposed in his absence for corruption.
His overthrow by the army heralded five years of political crises both in the Thai parliament and on the streets, where his elite-backed foes and his mainly working-class supporters have held crippling rival protests.
The situation escalated last year when more than 90 people, mostly civilians, were killed in clashes between the army and “Red Shirt” protesters who largely support Thaksin for his populist policies while in office.
Thaksin or his allies have won the most seats in the past four elections, but the courts reversed the results of the last two polls.
Thailand has also seen 18 actual or attempted military coups since it became a constitutional monarchy in 1932.
Yingluck has formed a six-party coalition, holding three-fifths of the seats in the lower house of parliament, in an attempt to bolster her hold on power.