The palace announced she died at Balmoral Castle, her summer residence in Scotland, where members of the royal family had rushed to her side after her health took a turn for the worse.
A link to the almost-vanished generation that fought World War II, she was the only monarch most Britons have ever known.
Her 73-year-old son Prince Charles automatically becomes king, though the coronation might not take place for months. It was not immediately known whether he will call himself King Charles III or some other name.
The BBC played the national anthem, “God Save the Queen,” over a portrait of her in full regalia as her death was announced, and the flag over Buckingham Palace was lowered to half-staff as the second Elizabethan age came to a close.
British Prime Minister Liz Truss, appointed by the queen just 48 hours earlier, pronounced the country “devastated” and called Elizabeth “the rock on which modern Britain was built.”
World leaders extended condolences and paid tribute to the queen.
In Canada, where the British monarch is the country’s head of state, Prime Minister Justin Trudeau saluted her “wisdom, compassion and warmth.” In India, once the “jewel in the crown” of the British empire, Prime Minister Narendra Modi tweeted: “She personified dignity and decency in public life. Pained by her demise.”
President Joe Biden called her a “stateswoman of unmatched dignity and constancy who deepened the bedrock alliance between the United Kingdom and the United States.”
Since Feb. 6, 1952, Elizabeth reigned over a Britain that rebuilt from a ruinous and financially exhausting war and lost its empire; joined the European Union and then left it; and made the painful transition into the 21st century.
She endured through 15 prime ministers, from Winston Churchill to Truss, becoming an institution and an icon — a fixed point and a reassuring presence even for those who ignored or loathed the monarchy.
She became less visible in her final years as age and frailty curtailed many public appearances. But she remained firmly in control of the monarchy and at the center of national life as Britain celebrated her Platinum Jubilee with days of parties and pageants in June.