Henry William Blair (December 6, 1834March 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.