Foreign Secretary William Hague paid tribute to Mr Reagan at the ceremony at the US Embassy in London this morning.
He said: "It is a great honour for me personally to take part in a ceremony for a man who changed the political landscape at the time I first became involved in it."
He was joined by former US secretary of state Condoleezza Rice as the 10-foot bronze figure was revealed.
Mr Hague said: "He joins the ranks of great men and women whose statues adorn our London streets; Nelson, Wellington, Lincoln, Churchill, Roosevelt, Edith Cavell and Nelson Mandela.
"Statues bring us to face to face with our heroes long after they are gone.
"Ronald Reagan is without question a great American hero; one of America's finest sons, and a giant of 20th-century history. You may be sure that the people of London will take this statue to their hearts.
Mr Hague said: "She has asked me to say these words to you: Ronald Reagan was a great president and a great man - a true leader for our times. He held clear principles and acted upon them with purpose.
"It was a pleasure to be his colleague and his friend, and I hope that this statue will be a reminder to future generations of the debt we owe him."
The statue was commissioned by the Reagan Memorial Fund Trust.