Theodore Sedgwick Fay (February 10, 1807 - November 17, 1898) was a writer from the United States who spent much of his life in Germany.
Fay initially worked as a clerk for his father, an attorney. His father died in 1825, and he continued long enough in law to be admitted to the bar in 1828, but he quickly left the legal profession for periodical journalism, where he made a name for himself for some years. To this period belongs his book Dreams and Reveries of a Quiet Man (2 vols., 1832), which contained papers he had written for the New York Mirror, where he was an editor beginning in 1828. He married Laura Gardenier in 1833, and went traveling in Europe for three years, while sending back articles to the Mirror. He served with ability in the United States diplomatic service, first as secretary of the legation at London briefly (1837), then at Berlin (1837–53), and next (1853–61) as Minister at Berne, Switzerland. He retired from his diplomatic career in 1861. He then moved to Berlin. His first wife died while he was at Berne, and he later married a German woman.