According to Wiki:
Governor Thomas Dorr 1844, convicted of treason against the state of Rhode Island; see Dorr Rebellion; released in 1845; civil rights restored in 1851; verdict annulled in 1854.
John Brown, convicted of treason against the Commonwealth of Virginia in 1859 for attempting to organize armed resistance to slavery
Aaron Dwight Stevens, took part in John Brown's raid and was executed for treason against Virginia
William Bruce Mumford 1862 convicted of treason and hanged for tearing down a United States flag during the American Civil War.
David Owen Dodd 1864 hanged for treason as a Confederate spy during the American Civil War.
Mary Surratt, Lewis Powell, David Herold, and George Atzerodt in 1865 hanged for treason and conspiracy for the Lincoln assassination and conspiracy - by military tribunal.
Iva Toguri D'Aquino, who is frequently identified with "Tokyo Rose" convicted 1949. Subsequently pardoned by President Gerald Ford
Herbert Hans Haupt German-born naturalized U.S. citizen, in 1942 was convicted of treason and executed for giving aid and comfort to the enemy (his son) and for espionage.
Martin James Monti, United States Army Air Force pilot, convicted of treason for defecting to the Waffen SS in 1944
Robert Henry Best, convicted of treason on April 16, 1948 and served a life sentence
Mildred Gillars, "Axis Sally," convicted of treason on March 8, 1949, served 12 years of a 10- to 30-year prison sentence
Tomoya Kawakita, sentenced to death for treason in 1952, but eventually released by President John F. Kennedy to be deported to Japan