Alexander Stewart (26 March 1746 – August 1831) was an Irish landowner and Member of Parliament.
He was the son of Alexander Stewart of Mount Stewart by his wife Mary, sister and heiress of Robert Cowan of Bombay. His elder brother was Robert Stewart, 1st Marquess of Londonderry.
On 1 July 1814 his nephew Sir Charles Stewart, the MP for County Londonderry, was raised to the peerage as Baron Stewart, and Alexander Stewart was elected to replace him on 19 July. He sat for the constituency until the 1818 general election, when his son was elected instead.
On 2 October 1791 Stewart was married to Mary Moore, daughter of Charles Moore, 1st Marquess of Drogheda and granddaughter of Francis Seymour-Conway, 1st Marquess of Hertford. Mary's aunt Sarah Frances Seymour-Conway was the first wife of Alexander's brother Lord Londonderry. Their children included the son and heir Alexander Robert Stewart.
Alexander Stewart lived at Ards House, Dunfanaghy, County Donegal, which he purchased in 1782. His family papers are in the Public Record Office of Northern Ireland.