William Roscoe Thayer (January 16, 1859 - September 7, 1923) was an American author and editor who wrote about Italian history. Thayer was born in Boston, Massachusetts. He studied at St. Mark's Academy, Concord, New Hampshire, traveled with a private tutor in Europe, and graduated from Harvard University in 1881, in the class with Theodore Roosevelt. For several years, he was assistant editor of the Philadelphia Evening Bulletin. He then returned to Harvard, receiving the degree of A. M. in 1886. He was editor of the Harvard Graduates' Magazine from its foundation in 1892 until 1915. In 1903, at the International Historical Congress at Rome, he represented both Harvard University and the American Historical Association, and in 1906 was their representative at the Italian Historical Congress in Milan.