The Royal Court said in a statement broadcast across the kingdom that the king had died early Friday. The royal court did not disclose the exact cause of death. An announcement quoted by the official Saudi Press Agency said the king had a lung infection when he was admitted on Dec. 31 to a Riyadh hospital.
The king's death adds yet another element of uncertainty in a region already overwhelmed by crises and as Saudi Arabia is itself in a struggle with Iran for regional dominance.
The royal family moved quickly to assure a smooth transition of power in a nation that is a close ally of the United States, the world's largest exporter of oil and the religious center of the Islamic faith. In a televised statement, Abdullah's brother, Crown Prince Salman, announced that the king had died and that he had assumed the throne.
Salman's ascension appears to signal that the kingdom will preserve its current policies, but he faces exceptional new challenges. Though Saudi Arabia has traditionally preferred to push its agenda through checkbook diplomacy, it has taken a far more muscular approach since the Arab Spring, offering generous support to its allies, like Egypt, while working to oppose adversaries like President Bashar Assad of Syria and the Muslim Brotherhood.
Even as the drop in the price of oil has depleted its own treasury, it has steadfastly refused to cut the supply, hoping to increase market share at the expense of adversaries that are less able to pump oil at low prices.
"As our countries worked together to confront many challenges, I always valued King Abdullah's perspective and appreciated our genuine and warm friendship,'' President Barack Obama said in a statement issued by the White House. "As a leader, he was always candid and had the courage of his convictions."
Vice President Joe Biden announced that he was to lead the U.S. delegation "to pay our respects and offer condolences."
Accidents of birth and geology made Abdullah one of the world's wealthiest and most powerful men. In control of a fifth of the world's known petroleum reserves, he traveled to medical appointments abroad in a fleet of jumbo jets, and the changes he wrought in Saudi society were fueled by gushers of oil money.
As king he also bore the title of custodian of Islam's holiest sites, Mecca and Medina, making him one of the faith's most important figures.
Abdullah had grown accustomed to the levers of power long before his ascension to the throne in August 2005. After his predecessor, King Fahd, a half brother, had a stroke in November 1995, Abdullah, then the crown prince, ruled in the king's name.
Yet Abdullah spoke as plainly as the Bedouin tribesmen with whom he had been sent to live in his youth. He refused to be called "your majesty" and discouraged commoners from kissing his hand. He shocked the 7,000 or so Saudi princes and princesses by cutting their allowances. He was described as ascetic, or as ascetic as someone in the habit of renting out entire hotels could be.
Abdullah's reign was a constant effort to balance desert traditions with the demands of the modern world, making him appear at times to be shifting from one to the other.
When popular movements and insurgencies overthrew or threatened long-established Arab rulers from Tunisia to Yemen in 2011, he reacted swiftly.
On his return from three months of treatment for a herniated disk and a blood clot in New York and Morocco, his government spent $130 billion to build 500,000 units of low-income housing, to bolster the salaries of government employees and to ensure the loyalty of religious organizations.
He also created a Facebook page, where citizens were invited to present their grievances directly to him, although it was not known how many entries actually reached him.
But in at least two telephone calls he castigated Obama for encouraging democracy in the Middle East, saying it was dangerous. And he showed no tolerance for the sort of dissent unfolding elsewhere.
The grand mufti, the kingdom's highest religious official, proclaimed that Islam forbade street protests. Scores of protesters who failed to heed that message were arrested in the chiefly Shiite eastern provinces. A new law imposed crippling fines for offenses, like threatening national security, that could be broadly interpreted.
Reaching beyond his borders, Abdullah sent tanks to help quell an uprising in neighboring Bahrain.
Moves of moderation
Still, Abdullah became, in some ways, a force of moderation. He contested al-Qaida's militant interpretations of the faith as justifying, even compelling, terrorist acts. He ordered that textbooks be purged of their most extreme language and sent 900 imams to re-education sessions. He had hundreds of militants arrested and some beheaded.
But he was also mindful that his family had, since the 18th century, derived its authority from an alliance with the strict Wahhabi sect of Sunni Islam. He accordingly made only modest changes to the kingdom's conservative clerical establishment. When Islamic State forces conquered vast stretches of Syria and Iraq, imposing a creed linked to Saudi Arabia's own, the kingdom was slow to respond.
However, Abdullah chastised senior clerics for not speaking out more forcibly against the jihadists, and he eventually sent Saudi pilots to participate in a U.S.-led campaign against the Islamic State.
Abdullah's Saudi Arabia had hurtled from tribal pastoralism to advanced capitalism in little more than a generation. The fundamentalist clerics who gave the family legitimacy remained a powerful force. Women who appeared in public without the required covering risked arrest or a beating from the religious police.
Abdullah did make changes that were seen as important in the Saudi context. He allowed women to work as supermarket cashiers and appointed a woman as a deputy minister. At the $12.5 billion research university he built and named for himself, women study beside men.
However, he did not fulfill a promise made to Barbara Walters of ABC News in his first televised interview as king in October 2005: that he would allow women to drive, a hugely contentious issue in Saudi Arabia.
Although he ordered the kingdom's first elections for municipal councils in 2005, a promised second election, in October 2009, in which women would vote, was postponed until September 2011. Then in March of that year, the Ministry of Municipal and Rural Affairs announced that the question of women voting would be put off indefinitely "because of the kingdom's social customs."
Abdullah's greatest legacy, however, may prove to be a scholarship program that sent tens of thousands of young Saudi men and women abroad to study at Western universities and colleges. It has been suggested that the changes long resisted by conservative forces - resistance that even a king could not overcome - would one day come about as those men and women rose in the government, industry and academia.
Perhaps Abdullah's most daunting challenge arrived in the wake of the Sept. 11, 2001, terrorist attacks, with the revelation that 15 of the 19 hijackers were Saudis. The royal family at first railed at what it called a vicious smear campaign against the kingdom, then ruthlessly suppressed known militants - not least because the monarchy itself was a main target of al-Qaida.
Striking a balance was almost always Abdullah's preference. He strove to keep oil prices high, but not so high that they prompted consumers to abandon petroleum, then hedged his bets by investing billions in solar energy research. In 2008, he convened a meeting of world religious leaders to promote tolerance, but held it in Madrid rather than Saudi Arabia, where the public practice of religions other than Islam is outlawed.
Yet Abdullah could, and did, take strong positions. He denounced the U.S.-led invasion of Iraq as "an illegal occupation"; proposed a comprehensive peace plan for the Middle East that included recognition of Israel by Arab nations; and urged in a secret cable that the United States attack Iran, Saudi Arabia's great rival.
"Cut off the head off the snake," he said.
His kingdom's interests always came first. Although U.S. oil companies discovered and developed the Saudi oil fields, he cut deals with Russian, Chinese and European petroleum companies. He made it clear that the world's energy appetites mattered less than Saudi Arabia's future.