William Allen (August 13, 1827 – July 6, 1881) was an United States Representative from Ohio during the early part of the American Civil War.
Allen was born near Hamilton, Ohio, where he attended the public schools. As a young man, he taught school, then studied law. Allen was admitted to the bar in 1849 and commenced practice in Greenville, Ohio, in 1850. He was the prosecuting attorney of Darke County from 1850 until 1854.
Allen was elected as a Democrat to the Thirty-sixth and Thirty-seventh Congresses (March 4, 1859–March 3, 1863), where he served as chairman, Committee on Expenditures in the Department of the Interior (Thirty-seventh Congress). He declined to be a candidate for renomination in 1862 and resumed the practice of law. He became affiliated with the Republican Party at the close of the Civil War and was appointed judge of the Court of Common Pleas of the second judicial district in 1865. He declined the Republican nomination for election to the Forty-sixth Congress in 1878 because of failing health. He was interested in banking until his death in Greenville, Ohio in 1881. He was buried in Greenville Cemetery.