William Sydney Thayer (June 23, 1864 – December 10, 1932) was a physician and professor of medicine at the Johns Hopkins Hospital and Medical School.

Thayer was born in Milton, Massachusetts, where his father James Bradley was a professor of law at Harvard University.

A younger sibling, Ezra, became dean of the Harvard Law School.

William was educated at Harvard University, receiving a BA degree in 1885 after being briefly suspended for a prank.

[3][4] Thayer married Susan Chisolm Read, a student of nursing from Johns Hopkins, in 1901.