Robert Helyer Thayer (September 22, 1901 − January 26, 1984) was an American lawyer, naval officer and diplomat.
Thayer was born in Southborough, Massachusetts, the son of Rev. William Greenough Thayer (1863−1934), headmaster of St. Mark's School from 1894−1930, and Violet Otis Thayer (1871−1962). He attended St. Mark's School, then studied for degrees from Amherst College and Harvard University. His elder brother, Sigourney Thayer (1896−1944), was a theatrical producer, aviator and poet.
Thayer practiced law in New York City under Gen William Joseph Donovan.
Thayer assisted Charles Lindbergh's lead lawyer, Henry Skillman Breckinridge on the famous Lindbergh kidnapping case in 1932, staying at the Charles A Lindbergh residence in Hopewell, New Jersey, until the body of the child was found on 12 May 1932. Bruno Richard Hauptmann was executed for this crime in 1936.
During World War II, Thayer was commissioned in the US Navy. He was an intelligence officer in the South Pacific early in the war and then went to Europe, where he took part in the invasions of Normandy and southern France. He returned to the Pacific in time for the invasion of the Philippines.
In 1945, he was an assistant to John Foster Dulles, who became secretary of state in the Dwight D. Eisenhower administration, at the organizing conference of the United Nations at San Francisco.