Sir Donald Macmaster, 1st Baronet, KC (3 September 1846 – 3 March 1922) was a Canadian lawyer and a politician in both Canada and the United Kingdom.

Macmaster was born into a family of Scottish descent in Williamstown, Glengarry County, Canada West (now in eastern Ontario). During the Fenian Raids in 1866 he served as a lieutenant in the Williamstown Volunteer Infantry. He studied law at McGill University, was called to the Quebec bar in 1871, and set up practice in Montreal. Macmaster served as Crown Prosecutor for many cases. He was called to the Ontario bar and appointed Queen's Counsel in 1882.

He represented Glengarry in the Legislative Assembly of Ontario from 1879 to 1882 and in the House of Commons of Canada as a Conservative member from 1883 to 1887, when he lost his seat.

In 1905, Macmaster emigrated to the United Kingdom and settled in London, intending to practise in Privy Council cases, in which he already had considerable experience. Having been defeated at Leigh in 1906, in 1910 he was elected to the House of Commons as Conservative member for the Chertsey division of Surrey, holding the seat until his death. He was created a baronet in the 1921 New Year Honours.^[1]

In 1880, he married Janet Macdonald, who died less than three years later. In 1890 he married the American Ella Virginia DeFord. Their only son, Donald, was killed in action at the Battle of Loos while commanding a company of the Cameron Highlanders on 25 September 1915.