Louis Arthur Johnson was an American politician and attorney who served as the second United States Secretary of Defense from 1949 to 1950. He was the Assistant Secretary of War from 1937 to 1940 and the 15th national commander of the American Legion from 1932 to 1933.

The United States secretary of defense (SecDef) is the head of the United States Department of Defense, the executive department of the U.S. Armed Forces, and is a high-ranking member of the federal cabinet. The secretary of defense's position of command and authority over the military is second only to that of the president of the United States, who is the commander-in-chief. This position corresponds to what is generally known as a defense minister in many other countries. The secretary of defense is appointed by the president with the advice and consent of the Senate, and is by custom a member of the Cabinet and by law a member of the National Security Council.