| Honorific-Prefix: | The Honourable |
| Alexander Stirling MacMillan | |
| Birth Date: | 31 October 1870 |
| Birth Place: | Upper South River, Nova Scotia |
| Death Place: | Halifax, Nova Scotia |
| Office: | 13th Premier of Nova Scotia |
| Term Start: | July 10, 1940 |
| Term End: | September 8, 1945 |
| Lieutenant Governor: | Frederick F. Mathers Henry Ernest Kendall |
| Predecessor: | Angus L. Macdonald |
| Successor: | Angus L. Macdonald |
| Office1: | Member of Legislative Council |
| Term Start1: | June 25, 1925 |
| Term End1: | September 5, 1928 |
| Office2: | MLA for Digby County |
| Term Start2: | October 1, 1928 |
| Term End2: | August 22, 1933 |
| Predecessor2: | William Hudson Farnham John Louis Philip Robicheau |
| Successor2: | Joseph Willie Comeau |
| Alongside2: | Joseph Willie Comeau |
| Office3: | MLA for Hants |
| Term Start3: | August 22, 1933 |
| Term End3: | October 23, 1945 |
| Predecessor3: | Albert E. Parsons Edgar Nelson Rhodes |
| Successor3: | Robert A. MacLellan |
| Occupation: | businessman |
| Party: | Liberal |
Alexander S. MacMillan (October 31, 1870 – August 7, 1955) was a Nova Scotia politician and businessman, the 13th premier of Nova Scotia, from 1940 to 1945.
MacMillan was born in Upper South River in Antigonish County. He made his fortune in lumbering and construction before being made chairman of the Nova Scotia Highways Board in 1920 and serving briefly as minister of highways in 1925. He was a member of Nova Scotia's appointed upper house, the Legislative Council from 1925 until 1928 when he won a seat in the Nova Scotia House of Assembly as a Liberal. He again became minister of highways in 1933. In 1940, when Premier Angus L. Macdonald went to Ottawa to serve in the wartime cabinet of William Lyon Mackenzie King, MacMillan became premier in his place. He retired as premier and from politics in 1945 to allow Macdonald to resume his provincial career. MacMillan died in Halifax at the age of 83.