Thomas Hay, 9th Earl of Kinnoull PC (4 July 1710 – 27 December 1787), styled Viscount Dupplin from 1719 to 1758, was a Scottish peer, British politician, and scholar.

In 1751, Horace Walpole described the earl as "fond of forms and trifles," but "not absolutely a bad speaker.

After the Duke of Newcastle resigned his posts in 1762, Kinnoull told George III he would support whatever minister the King should name.

The 9th Earl built a modest castle near the highest point of Kinnoull Hill, its tower overlooking the Tay.

Jane Austen described Kinnoull Tower in Lesley Castle, a story she wrote in 1790, the year after she stayed there with D'Arcy Wentworth, during their ramble through Scotland :[5][6][7] I continue secluded from Mankind in our old and Mouldering Castle, which is situated two miles from Perth on a bold projecting rock, and commands an extensive view of the Town and its delightful Environs... You can form no idea sufficiently hideous, of its dungeon like form.

Another of the 9th Earl's lasting legacies is the Perth Bridge over the River Tay, "the Auld Brig" in the local Scots dialect, which he helped fund.

Duplin County, North Carolina, was named for Viscount Dupplin, as the 9th Earl was known when he served on the Board of Trade and Plantations in the 1740s.