| Honorific-Prefix: | The Honourable |
| Thomas Young Duncan | |
| Order: | 3rd Minister of Agriculture |
| Term Start: | 2 July 1900 |
| Term End: | 6 August 1906 |
| Primeminister: | Richard Seddon |
| Predecessor: | John McKenzie |
| Successor: | Robert McNab |
| Constituency Mp1: | Oamaru |
| Parliament1: | New Zealand |
| Term Start1: | 5 December 1890 |
| Term End1: | 7 December 1911 |
| Predecessor1: | Thomas William Hislop |
| Successor1: | Ernest Lee |
| Office2: | New Zealand Legislative Councillor |
| Appointer2: | Joseph Ward |
| Term Start2: | 13 June 1912 |
| Term End2: | 18 August 1914† |
| Birth Date: | 1836 |
| Birth Place: | Plumbridge, Ireland |
| Death Date: | 18 August 1914 |
| Death Place: | New Zealand |
| Party: | Liberal |
Thomas Young Duncan (1836 – 18 August 1914), sometimes referred to as "Tam Duncan",[1] was a New Zealand politician of the Liberal Party.
Born at Plumbridge, County Tyrone, Ireland, in 1836, Duncan was educated at Castledamph National School. In 1858 he went to Victoria where he worked on the goldfields, and then in 1862 followed the gold rush to Central Otago in New Zealand. After little success, he began farming at Pukeuri, north of Oamaru, and lived there for the remainder of his life.
He represented the Waitaki electorate from 1881 to 1890 and then the Oamaru electorate from 1890 to 1911, when he was defeated by Ernest Lee. He was appointed to the New Zealand Legislative Council on 13 June 1912 and served until his death in 1914.