Sir Thomas Lachlan (Tom) Macdonald KCMG (14 December 1898 – 11 April 1980) was a New Zealand politician of the National Party. He served in both World Wars. He was a High Commissioner to the United Kingdom.
He was born in Invercargill in 1898. His parents were Thomas Forsaith Macdonald, a farmer, and Margaret Ann Matheson. One of his great-grandfathers, Thomas Forsaith, was a member of the 1st New Zealand Parliament. Macdonald was educated at South School and Southland Boys' High School. He was in the NZEF in Palestine in World War I, and in World War II he served in Egypt, rising to the rank of captain until he was invalided home in 1943.
He was the Member of Parliament for Mataura 1938 to 1946, then Wallace 1946 to 1957, when he retired. He was Minister of Defence (1949–1957), Minister of External Affairs (1954–1957), and Minister of Island Territories (1955–1957) in the First National Government. From 1961 to 1967 he was the New Zealand High Commissioner to the United Kingdom.