Lucien Bonaparte Caswell (November 27, 1827 – April 26, 1919) was an American lawyer and Republican politician.

When he was nine, he moved with his family to the frontier Wisconsin Territory and settled along the Rock River, just south of Lake Koshkonong.

He began to study law in Beloit with the practice of future United States Senator Matthew H. Carpenter.

He represented Wisconsin's 2nd congressional district, covering Columbia, Dane, Jefferson, and Sauk counties.

As a representative, he was active in establishing the Federal appeals court system and overseeing the construction of the Library of Congress.