The Prime Minister said the death of Elizabeth, aged 96, was a “huge shock” to the nation and to the world.
Speaking in Downing Street, Ms Truss described the Queen as the “very spirit of Great Britain” before declaring: “That spirit will endure.”
She also said: “Queen Elizabeth II was the rock on which modern Britain was built. Our country has grown and flourished under her reign.
“Britain is the great country it is today because of her.”
Ms Truss added: “Through thick and thin, Queen Elizabeth II provided us with the stability and the strength that we needed.”
Ms Truss, who met the Queen on Tuesday to be confirmed as the new Prime Minister, paid tribute to the Queen’s “dignity and grace” during her 70-year reign.
She was informed of the Queen’s death at 4.30pm as she worked in Downing Street, with the news broken by Cabinet Secretary Simon Case.
Ms Truss said: “In the difficult days ahead, we will come together with our friends, across the United Kingdom, the Commonwealth and the world, to celebrate her extraordinary lifetime of service.
“It is a day of great loss, but Queen Elizabeth II leaves a great legacy.
“Today the Crown passes – as it is has done for more than a thousand years – to our new monarch, our new head of state: His Majesty King Charles III.
“With the King’s family, we mourn the loss of his mother. And as we mourn, we must come together as a people to support him.
“To help him bear the awesome responsibility that he now carries for us all.
“We offer him our loyalty and devotion just as his mother devoted so much to so many for so long.
“And with the passing of the second Elizabethan age, we usher in a new era in the magnificent history of our great country, exactly as Her Majesty would have wished, by saying the words God save the King.”
After making her statement the new Prime Minister spoke to the new King from No 10.
Labour leader Sir Keir Starmer described the Queen as a “remarkable sovereign”, adding: “It is a deep, private loss for the royal family and all our thoughts are with them at this time. The nation shares in their grief.
“We will always treasure Queen Elizabeth II’s life of service and devotion to our nation and the Commonwealth; our longest-serving and greatest monarch.
“Above the clashes of politics, she stood not for what the nation fought over, but what it agreed upon. As Britain changed rapidly around her, this dedication became the still point of our turning world.
“So as our great Elizabethan era comes to an end, we will honour the late Queen’s memory by keeping alive the values of public service she embodied.
“For 70 years, Queen Elizabeth II stood as the head of our country. But, in spirit, she stood amongst us.”