The United States under secretary of the Army is the second-highest-ranking civilian official of the United States Department of the Army, serving directly under the United States Secretary of the Army. The Secretary and Under Secretary, together with two military officers, the Chief of Staff of the United States Army and the Vice Chief of Staff of the United States Army, constitute the senior leaders of the United States Army.

The secretary of the Army is a senior civilian official within the United States Department of Defense, with statutory responsibility for all matters relating to the United States Army: manpower, personnel, reserve affairs, installations, environmental issues, weapons systems and equipment acquisition, communications and financial management.