Alexander Seton Montgomerie, 9th Earl of Eglinton (ca. 1660 – February 18, 1729) was a Scottish peer.
Although from a traditionally Roman Catholic family, Eglinton became one of the first firmly Presbyterian nobles. During the English Civil War he was colonel of horse in the army supporting the English parliamentarians against King Charles II. He was present at the siege of York in April–June and the battle of Marston Moor on 2 July. Later, Eglinton joined Argyll and other members of the moderate kirk party and supported the King. Later, he was one of the Scottish shire commissioners selected to negotiate with Monck in 1659 about the restoration of King Charles and lived to see the restoration of the king.
He married first Margaret Cochrane, daughter of William Cochrane, Lord Cochrane. He married second Lady Ann Gordon, daughter of George Gordon, 1st Earl of Aberdeen and Anne Lockhart of Tarbrax, Countess of Aberdeen. He married third Susanna Kennedy, daughter of Sir Archibald Kennedy, 1st Baronet.