RIYADH—King Abdullah bin Abdulaziz al Saud, who insulated Saudi Arabia from the Islamist and democratic forces roiling the Middle East in the name of stability, died early Friday at about age 90, the royal court said.

Abdullah’s half-brother, Crown Prince Salman, who is 79 years old, was declared king and Prince Muqrin, 69, became crown prince, according to the statement read on state television.

Born before his father founded the modern Saudi state in 1932, Abdullah focused his final years on internal and external security threats to a nation he had seen grow into an oil giant and center of political and religious power in the region.

In one of his final statements, an address this month before the consultative Shoura Council read by his half-brother and heir to the throne, Abdullah emphasized that his country was “blessed with security and stability” in the heart of a volatile region.

For Abdullah, who is widely believed to have run Saudi Arabia for the decade before his accession to the throne in 2005 following the death of his stroke-disabled brother King Fahd, stability was the paramount virtue. He abhorred the revolutions that brought religious fundamentalists to power in Iran in 1979 and unseated longtime fellow rulers in Egypt, Libya, and Tunisia three decades later.