Prince Salman bin Sultan bin Abdul Aziz was removed from his post as deputy defense minister " upon his request, " SPA said, citing a royal decree.
He was replaced by Prince Khaled bin Bandar bin Abdul Aziz, the governor of Riyadh.
Saudi King also appointed his son Prince Turki as governor of Riyadh region, SPA said.
The outgoing deputy minister is a son of the late Crown Prince Sultan, who served as a defense minister for nearly five decades.
SPA said the king also removed the chief of staff, General Hussein al - Qabeel, who was retiring, and replaced him with his deputy. General Fayad al - Rawyli.
The defense overhaul came a month after the king removed the oil - rich Persian Gulf state ' s powerful intelligence chief, Prince Bandar bin Sultan.
A power struggle is under way in Saudi Arabia since King Abdullah is in very frail health.
According to an article entitled The Struggle for Power in Saudi Arabia published in Foreign Policy on previous June, “With the reign of King Abdullah in its twilight, Saudi Arabia has become consumed with speculation about the future of the ruling al - Saud brotherhood. ”
The article also said that in recent months, King Abdullah did his best to pave the way for his son Miteb, 61, to become the king of the oil - rich country.
The article noted that the leadership turnover in the Arab country is taking place in a critical time and “Saudi leaders have been deeply anxious about the waning of American leadership in the Middle East. ”
The 89 - year - old king’s health has declined over the past few years, during which he has been hospitalized several times.