Sir Robert Temple Armstrong, Baron Armstrong of Ilminster CB CVO KCB GCB (30 March 1927— ) is a British life peer and former civil servant.

In a long civil service career, Armstrong worked in several departments, including HM Treasury and the Home Office. From 1979 to 1987, he served as Secretary of the Cabinet . He was subsequently made a life peer as Baron Armstrong of Ilminster, of Ashill in the County of Somerset in 1988, and sits as a crossbencher.