Harman Blennerhassett (8 October 1764 – 2 February 1831) was an Anglo-Irish lawyer, a member of the Society of United Irishmen who emigrated in advance of their rebellion in 1798 to become a socially and politically distinguished plantation owner in then-western Virginia.

Implicated in the Burr conspiracy, an alleged military plot with Britain to separate the Louisiana Territory from the American Union, he was twice arrested and financially ruined.

Blennerhassett visited Paris in 1790; inherited the family estate in 1792; joined the secret Society of United Irishmen in 1793, which initially dedicated itself to reform, but later turned militantly radical; and in 1794 married Margaret Agnew, daughter of his sister Catherine and Major Robert Agnew, a career officer in the British army.

His three stays on the island resulted in its becoming headquarters for his mysterious 1806–1807 military expedition to the Southwest, an alleged scheme to separate the Louisiana Territory from the American union with the assistance of the British.

[4] Now forced to earn a living for himself and family, Blennerhassett first settled on a cotton plantation near Port Gibson, Mississippi, where he lost what was left of his once large fortune.