Henry W. Blair (December 6, 1834 - March 14, 1920) was a United States Representative and Senator from New Hampshire. Born in Campton, he attended the common schools and private academies, studied law, was admitted to the bar in 1859 and commenced practice in Plymouth. He was appointed prosecuting attorney for Grafton County in 1860, and during the Civil War he served in the Union Army as lieutenant colonel of the Fifteenth Regiment, New Hampshire Volunteer Infantry.
Blair was a member of the New Hampshire House of Representatives in 1866 and a member of the New Hampshire Senate in 1867-1868. He was elected as a Republican to the Forty-fourth and Forty-fifth Congresses (March 4, 1875-March 3, 1879); he was not a candidate for renomination in 1878 but was elected to the U.S. Senate on June 17, 1879, for the vacancy in the term ending March 3, 1885, and served from June 20, 1879, to March 3, 1885. The State legislature not being in session, he was appointed on March 5, 1885, and elected on June 17, 1885, to fill the vacancy in the term beginning March 4, 1885, and served from March 10, 1885, to March 3, 1891. He was an unsuccessful candidate for renomination in 1891. While in the Senate, he was chairman of the Committee on Education and Labor (Forty-seventh through Fifty-first Congresses).
He declined an appointment as judge of the United States District Court for the district of New Hampshire tendered by President Benjamin Harrison in 1891, and was appointed Envoy Extraordinary and Minister Plenipotentiary to China on March 6, 1891. He was objected to by the Chinese Government as being persona non grata, and subsequently tendered his resignation which was accepted October 6, 1891. He was elected to the U.S. House of Representatives (March 4, 1893-March 3, 1895) and was not a candidate for reelection in 1894. He engaged in the practice of law in Washington, D.C., until his death in 1920; interment was in Campton Cemetery.