General Sir Alexander Duff GCH (1777 – 21 March 1851) was a British Army officer of the Napoleonic era.
Duff was the second son of Alexander Duff, 3rd Earl Fife and the younger brother of James Duff, 4th Earl Fife. In 1793, he was commissioned an ensign in the 66th Regiment of Foot, and served at Gibraltar, in Flanders, in the East Indies in 1798, and under Baird during the invasion of Egypt. In 1806, Duff commanded the centre column in the attack on Buenos Aires.
He was appointed colonel of the 92nd Regiment of Foot in 1823, transferring to 37th Regiment of Foot in 1831. He was made GCH and knighted in 1834, and was promoted full general on 28 June 1838.
He was elected the Member of Parliament for Elgin Burghs, sitting from 1826 to 1831. From 1848 until his death he was Lord Lieutenant of Elginshire, and a deputy lieutenant of Banffshire. He lived at Delgatie Castle near Turriff, Aberdeenshire.
He died at Percy Cross, Walham Green, Fulham, Middlesex in March 1851. He had married Anne Stein, by whom he had two sons and two daughters, including James Duff, 5th Earl Fife and George Skene Duff.