Saudi Arabia's King Abdullah has appointed Prince Mohammed bin Nayef bin Abdul Aziz as minister of the interior.
Prince Mohammed, who is in his early 50s, replaces his uncle, Prince Ahmed bin Abdul Aziz, less than five months after he took up the post.
A decree said Prince Ahmed - one of the king's half-brothers - was relieved of his position "at his own request".
Prince Mohammed has for years been responsible for counter-terrorism activities at the interior ministry.
He is seen by Western powers as having effectively led the crackdown on Islamist militants in the Gulf state since 11 September 2001.
In 2009, the prince was targeted by a suicide bomber from al-Qaeda in the Arabian Peninsula, but escaped with only minor injuries.
He served 13 years as assistant minister under his father, Crown Prince Nayef, who was interior minister for 37 years until his death in June, and was then promoted to deputy minister under Prince Ahmed.
King Abdullah, Crown Prince Salman, Prince Ahmed and the late Prince Nayef are all sons of former King Abdul Aziz ibn Saud, who established the kingdom during his reign from 1932 to 1953. So far, five brothers have become kings and about 20 are still alive.
Prince Mohammed's appointment lifts him into a critical role that has until now only been held by the current ruling generation.
"I would assume he's from the second generation of princes who are more receptive to ideas of reform," Saudi commentator Jamal Khashoggi told the Reuters news agency.
"But he is good at making everybody think he is in their camp. That's what makes a successful politician," he added.
Columnist Hussein Shobokshi said the new interior minister was "perceived as progressive, efficient and result-orientated".
As interior minister, Prince Mohammed will control the police, a highly developed and well funded intelligence apparatus, numerous special forces units and elite counter-terrorism squads, border protection forces, critical installation protection forces and the religious police, known as the "mutawa".