Sir Thomas Lachlan Macdonald KCMG (14 December 1898 – 11 April 1980) was a New Zealand politician of the National Party.

One of his great-grandfathers, Thomas Forsaith, was a member of the 1st New Zealand Parliament.

He served in the New Zealand Expeditionary Force in Palestine in the First World War, and in the Second World War he served in Egypt, rising to the rank of captain until he was invalided home in 1943.

[2] Macdonald was the Member of Parliament for Mataura 1938 to 1946, then Wallace 1946 to 1957, when he retired.

[1] Mount Macdonald in Antarctica was named for him by the New Zealand Geological Survey Antarctic Expedition (1961–62).