The new minister is Prince Muhammed bin Nayef. He is the son of late Crown Prince Nayef Abdul-Aziz, who died in June.
Prince Nayef had also served as interior minister since 1975. He was credited with a harsh crackdown on al-Qaida elements in the kingdom in the early 2000s.
The official state news agency said Monday that outgoing Interior Minister Ahmed bin Abdul-Aziz asked to be relieved of his post, nearly five months after he was named. He is the king's half-brother.
Prince Muhammed escaped an al-Qaida assassination attempt in 2009. He served as commander of counterterrorism operations as well as deputy interior minister.