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'Arthur Hay, 9th Marquess of Tweeddale' (9 November 1824–29 December 1878), known before 1862 as 'Lord Arthur Hay' and between 1862 and 1876 as 'Viscount Walden', was a Scottish soldier and ornithologist. He was born at Yester, Gifford, East Lothian. He served as a soldier in India and the Crimea. He succeeded his father to the Marquessate in 1876. He died at Chislehurst, and was succeeded by his brother.

He was president of the Zoological Society of London. His ornithological works were published privately in 1881 by his nephew, Captain Robert George Wardlaw-Ramsay, with a memoir by Dr W. H. Russell, and the attribution 'Walden' is used in taxonomic listings.
He had a large private collection of birds, insects, reptiles and mammals. He employed Carl Bock to travel to the Malay archipelago and collect specimens. Tweeddale described about 40 species collected by Bock for the first time.