The Spanish tenor, who has sung more operatic roles than any tenor in history, told a gathering of dignitaries - including Secretary of State Colin Powell - he was "very happy, very honored and very proud" to receive the honor at a ceremony Monday night.
British Foreign Secretary Jack Straw, in Washington to meet with Powell on Tuesday, conferred the honor after the British ambassador to the United States, Christopher Meyer, lauded Domingo's unique talents.
"Placido's own motto is, 'If I rest I rust.' I, for one, have yet to see any signs of metal fatigue," Meyer said.
Domingo has sung 119 different operatic roles, ranging from Mozart and Verdi to Berlioz and Puccini. He is currently performing in the title role of Giordano's "Andrea Chenier" at the Metropolitan Opera in New York. Since 1996, he has served as artistic director of the Washington Opera.
Domingo, 61, said performing at London's Royal Opera House off and on for the past 30 years "has been among my greatest experiences."
Because he is not a British citizen, Domingo cannot use "Sir" in front of his name, but he can put the letters KBE after it. The initials stand for Knight Commander of the British Empire.
Americans who have received honorary knighthoods include U.S. Federal Reserve Chairman Alan Greenspan, former New York City Mayor Rudolph Giuliani, entertainer Bob Hope and former presidents George Bush and Ronald Reagan.
By Sam Hananel