William Flavius Lester Hadley (June 15, 1847 – April 25, 1901) was a U.S. Representative from Illinois.
Hadley was elected to the Fifty-fourth Congress in a special election and served from 1895 to 1897. He was an unsuccessful candidate for reelection in 1896. He died in Riverside, California, April 25, 1901. He is buried on an overlook in Woodlawn Cemetery.