Prince Mohammed bin Nayef.

RIYADH — Saudi Arabia’s interior minister, Prince Mohammed bin Nayef, a close friend of the United States and a scourge of Islamist militants, will be the country’s first king from the third generation of its ruling dynasty.

King Salman on Wednesday appointed the U.S. favorite and veteran security chief as crown prince, making him next-in-line to rule the world’s top oil exporter in a sudden reshuffle of top posts in the Al Saud dynasty.

By sending an assassin to try to kill Prince Mohammed when he was Saudi security chief in 2009, al-Qaeda paid him the compliment of treating him as one of its most dangerous enemies.

The prince narrowly survived that attack in which a militant approached him claiming he wanted to defect before detonating a bomb concealed under his clothes and was later named interior minister in November 2012.

The 55-year-old is now firmly established as the most powerful member of his generation in the ruling al-Saud family, and even before he becomes king he will be one of the most important figures in the kingdom.

He is the first grandson of Saudi Arabia’s founding monarch, King Abdulaziz, or Ibn Saud, to join the line of succession. Abdulaziz established the kingdom in 1932 and ruled until his death in 1953, a period in which Saudi Arabia assumed growing geopolitical importance because of its huge oil resources, developed with U.S. partners.

Mohammed remains in his position as interior minister, Wednesday’s decree said.

A March 2009 U.S. embassy cable released by WikiLeaks described Prince Mohammed as already being the de facto interior minister and said he was “held in high regard by Saudi King Abdullah … and well respected by the Saudi populace.”

Yet despite his constant contacts with Western – particularly American – officials and a prominent media presence through his security role, Prince Mohammed remains something of an unknown quantity.

Diplomats and Saudi analysts and academics are uncertain what positions he holds on the big long-term issue facing the kingdom: reconciling social change and a young population with conservative traditions and an oil-dependent economy.

Prince Mohammed is now the youngest member of the select group of princes at the top of government who control the most important portfolios, such as foreign affairs, intelligence and defense.

The reshuffle also saw Adel al-Jubeir become the first non-royal foreign minister of the kingdom. Jubeir succeeds Saud al-Faisal, the longest serving foreign minister in the world, who remained in his post for four decades. Al-Faisal has been appointed a special envoy of King Salman.

The kingdom’s new foreign minister is a U.S.-educated connoisseur of Washington’s diplomatic scene and longtime adviser to the kingdom’s rulers. He is also an articulate spokesman for his country’s new assertive approach to the Middle East’s growing conflicts.

Jubeir, 53, is not only a prominent public face of Saudi diplomacy, but also an insider in Riyadh.

As ambassador to Washington, he translated for former King Abdullah in meetings with President Obama and shuttled back and forth to the kingdom regularly to brief the king in person.