Thaddeus Reynolds Beal (1917-1981) was the United States Under Secretary of the Army from March 8, 1969, through September 21, 1971.

Born March 22, 1917, in New York City, Beal graduated from The Hotchkiss School in Connecticut and received a B.A.

He was president and chief executive officer, Harvard Trust Company of Cambridge, Massachusetts, 1957–1969.

He was active in business and community organizations in greater Boston area as trustee, Cambridge Savings Bank and Boston Personal Property Trust, director of Middlesex Mutual Insurance Company, member of Cambridge Redevelopment Authority, and trustee of Radcliffe College.

In 1972, after his term as Under Secretary of the Army ended, he resumed practice of law as a member of the firm of Herrick, Smith, Donald, Farley and Ketchum.