William Henry Hammett (March 25, 1799 – July 9, 1861) was an American clergyman and politician who served one term as a U.S. Representative from Mississippi from 1843 to 1845.
Born in Dunmanway, County Cork, Ireland, Hammett studied theology. He was chaplain of the University of Virginia at Charlottesville 1832–1834 and of the Virginia House of delegates.
Hammett was elected as a Democrat to the Twenty-eighth Congress (March 4, 1843 – March 3, 1845).
He died July 9, 1861, in Washington County, Mississippi.