Saudi Arabia's intelligence chief, Prince Bandar bin Sultan, has been removed from his post "at his own request", state media report.
Prince Bandar, King Abdullah's nephew and a former ambassador to the US, recently returned to Riyadh after two months abroad for medical treatment.
The 65 year old has been replaced by his deputy, Gen Youssef al-Idrissi.
His departure comes months after he was quoted warning of a "major shift" from the US over its Middle East policy.
Largely in protest over Washington's reluctance to get involved militarily in Syria, he reportedly told European diplomats in October that Saudi Arabia would be scaling back its co-operation with the CIA over arming and training rebel groups seeking to topple President Bashar al-Assad.
A trip to Moscow to press Russian President Vladimir Putin to abandon his support for the Syrian government also failed to produce results.
"He had been more or less disengaged from the Syrian file for the past five months," Mustafa Alani, a security expert with close ties to the Saudi government, told the Reuters news agency.
"The responsibility was divided between a number of people - officers in the intelligence sphere and other princes. So the reality is that any changes have already happened."
Interior Minister Prince Mohammed bin Nayef is believed to have taken overall control of the kingdom's Syria policy and attended a meeting of Western and Arab intelligence agencies in Washington in February.
As head of the General Intelligence Presidency, Prince Bandar was also said to have been closely involved in supporting Egypt's military-backed interim government after President Mohammed Morsi was ousted in July.
The royal decree quoted by the Saudi Press Agency on Monday did not say if the prince would continue as head of the National Security Council.