William Roscoe Thayer (January 16, 1859 – September 7, 1923) was an American author and editor who wrote about Italian history.

He studied at St. Mark's Academy, Concord, New Hampshire, traveled with a private tutor in Europe, and graduated from Harvard University in 1881.

For several years, he was assistant editor of the Philadelphia Evening Bulletin.

In 1914, he was elected to The American Academy of Arts and Letters and he received honorary degrees from Harvard, Yale, Brown and other universities.

Thayer served as a member of the Harvard Board of Overseers from 1913 until 1919.