Sir Alexander Stewart of Darnley (d. 1404) was a Scottish nobleman.
He was the son of Sir Alexander Stewart of Darnley. It is not known who his first wife was, but evidence that his son John Stewart of Darnley bore the emblems of Clan Turnbull as a quartering on his arms, suggests that she was of that blood. By this unknown spouse Darnley had six children:
Darnley, married secondly Janet Keith, widow of David Hamilton of Cadzow, and daughter of Sir William Keith of Galston, and was granted his wife's lands of Galston by John Stewart, Earl of Carrick. They had a son:
Balfour Paul, Sir James, Scots Peerage, IX vols. Edinburgh 1904.