William Henry Barnum (September 17, 1818 – April 30, 1889) was an American politician, serving as a state representative, congressman, U.S. senator, and finally as chairman of the Democratic National Committee.

The Barnum Richardson Company, of which he was chief executive, was headquartered in Lime Rock (now a neighborhood of Lakeville), and was the leading company in the Salisbury iron district of that time, owning or controlling iron mines, charcoal production resources, limestone quarries, and rail transportation.

The corporation grew to immense profitability and industrial volume, and became one of the foremost metal products manufacturers in the world.

William H. Barnum had a nephew, Milo Barnum Richardson, who was a leader in New York City finance and insurance, having acted as a founder of the New York City branch of the Caledonian Life Insurance Company of Scotland.

Although an Episcopalian (he was the principal donor for the construction of, and chairman of the incorporators of Trinity Episcopal Church in Lime Rock), he did not discriminate against Roman Catholics as so many in that area of New England did at the time.