William Hay, 10th Marquess of Tweeddale

William Montagu Hay, 10th Marquess of Tweeddale KT (29 January 1826 – 25 November 1911), known before 1878 as Lord William Hay or Lord William Montagu Hay, was a Scottish landowner, peer and politician. He was born at Yester House, near Gifford, East Lothian, and served in British India as a member of the Bengal Civil Service and later as a Liberal Member of Parliament.

In 1878 he succeeded his brother as Marquess of Tweeddale and as owner of some 40,000 acres in Scotland. He went on to become Lord High Commissioner to the General Assembly of the Church of Scotland and was appointed a Knight of the Thistle.

Hay was born at Yester House, the third son of George Hay, 8th Marquess of Tweeddale (1787–1876) by his marriage to Susan, a daughter of William Montagu, 5th Duke of Manchester. As a younger son, Hay was educated at the Imperial Service College and then served in the Bengal Civil Service from 1845 to 1862, including some years as Deputy Commissioner of Simla and then as Superintendent of the Hill States of Northern India.

Following his permanent return from India Hay was Liberal Member of Parliament for Taunton from 1865 to 1868, and was elected again for Haddington Burghs in 1878. He also became Chairman of the North British Railway Company.