Saudi Crown Prince Mohammed bin Salman, named heir to the region's most powerful throne last month, took temporary charge of the country Monday as the king left on holiday.
Prince Mohammed's father, King Salman, issued a royal decree deputising his son to "manage state affairs and guard the interests of the people" during the monarch's "personal break", state news agency SPA reported.
On June 21, King Salman stripped the title of crown prince from his nephew Mohammed bin Nayef, naming 31-year-old Prince Mohammed, often known as MBS, as heir to the throne.
The crown prince has earned a reputation as a reformist in the ultra-conservative kingdom but is seen by some as lacking experience.
Sunni-ruled Saudi Arabia, a key US ally and the world's largest oil exporter, heads an alliance that last month cut ties with Qatar over accusations Doha was too close to Saudi rival Iran and bankrolled Islamist extremist groups.
Qatar has denied the allegations.
The suspension of diplomatic and trade ties with Qatar has sparked the largest crisis to ever hit the Gulf Cooperation Council.