King Abdullah has already outlived two appointed successors from among the elderly group of sons of Saudi Arabia's founding monarch.
He named his half-brother, 76-year-old Defense Minister Prince Salman bin Abdul-Aziz, as Crown Prince last year to succeed him.
Salman had previously served for almost 50 years in the influential post of governor of Riyadh.
Before Sunday's appointment, the king's son Mishaal was governor of Najran, a southwestern mountainous region of Saudi Arabia that borders Yemen and has a significant population of Ismaili Shiites, an offshoot of the kingdom's Shia minority.
