RIYADH, Saudi Arabia — Saudi Arabia’s King Salman today appointed his 31-year-old son Mohammed bin Salman as crown prince, placing him first-in-line to the throne and removing the country’s counterterrorism czar and a figure well-known to Washington from the line of succession.

The monarch stripped Prince Mohammed bin Nayef from his title as crown prince and from his powerful position as the country’s interior minister overseeing security. The announcements were made in a series of royal decrees carried on the state-run Saudi Press Agency.

The all-but-certain takeover of the throne by Mohammed bin Salman awards near absolute powers to a prince who has ruled out dialogue with rival Iran, has moved to isolate neighboring Qatar for its support of Islamist groups and who has led a war in Yemen that has killed thousands of civilians.

The prince already oversees a vast portfolio as defense minister and is spearheading economic reforms. He has become popular among some of Saudi Arabia’s majority youth for pushing reforms that have opened the deeply conservative country to entertainment and greater foreign investments as part of an effort to overhaul the economy, including plans to list a percentage of the state-run oil giant Aramco.

The young prince was little known to Saudis and outsiders before Salman became king in January 2015. He had previously been in charge of his father’s royal court when Salman was the crown prince.

The Saudi monarch quickly awarded his son expansive powers and named him deputy crown prince two years ago to the surprise of many within the royal family who are more senior and more experienced than Mohammed bin Salman.

The appointment of such a young royal as the immediate heir to the throne essentially sets Saudi policy for decades and removes the challenge of uncertainty.