Saudi Arabian Prince Salman bin Abdel Aziz was appointed as defence minister in a royal decree issued on Saturday, DPA reported.
Prince Salman, believed to be 75 years old, has been the governor of the Riyadh Province since 1962.
The post is seen as the most important in the kingdom, which tries to enhance its relations with western allies partly through arms purchases worth billions of dollars.
His appointment comes two weeks after the death of his brother and predecessor Sultan bin Abdel Aziz in a New York hospital. Prince Sultan was crown prince as well as minister of defence.
Interior Minister Prince Naif was designated as the heir to the throne in the oil-producing kingdom.
Naif's son, Saud bin Naif, was appointed on Saturday as a special advisor to the crown prince.
The appointment of Naif, 78, has raised concern, with some pro-reform analysts saying the hardliner could halt reforms introduced by King Abdullah bin Abdul Aziz.