King Bhumibol Adulyadej of Thailand has endorsed Yingluck Shinawatra as prime minister, allowing her to formally take the post.
Ms Yingluck's party won a commanding victory in an election on 3 July. Parliament supported her candidacy as prime minister in a vote last week.
The BBC reports she leads a six-party coalition which dominates parliament.
Ms Yingluck, 44, is the youngest sister of Thaksin Shinawatra, who was ousted as prime minister in a 2006 coup.
Mr Thaksin now lives in Dubai. He dominated the election campaign and is said to control Pheu Thai, the party Ms Yingluck leads.
Ms Yingluck has denied her brother will control policy direction. She has promised to reduce economic hardship and reconcile Thailand's deep political divisions.