William Flavius Lester Hadley (June 15, 1847 – April 25, 1901) was a U.S. Representative from Illinois.

Born near Collinsville, Illinois, Hadley attended the common schools.

He was graduated from McKendree College, Lebanon, Illinois, in June 1867, and from the law department of the University of Michigan at Ann Arbor in 1871.

Hadley was elected as a Republican to the Fifty-fourth Congress to fill the vacancy caused by the death of Frederick Remann and served from December 2, 1895, to March 3, 1897.

This article incorporates public domain material from the Biographical Directory of the United States Congress