Crown Prince Al Soud, 81, was the deputy minister of aviation and defence, and was one of the top figures in the Saudi kingdom.
"With deep sorrow and sadness ... King Abdullah bin Abdul Aziz mourns the death of his brother and his Crown Prince Sultan who died at dawn this morning Saturday outside the kingdom following an illness," the Saudi state press agency said.
Saudi state television said Sultan's funeral would be held on Monday. With Sultan's death, half brother Prince Nayef bin Abdul-Aziz, the longtime minister of interior, becomes next in line to rule after King Abdullah.