William John Duncan Dempster (21 October 1876 – 25 October 1964) was a member of the North-West Mounted Police (NWMP) in the Yukon Territory during the early 20th century.
He gained notice for his involvement in finding the Lost Patrol, three NWMP officers and an ex-NWMP officer/guide who disappeared on the trail between Fort McPherson and Dawson City in the winter of 1910–11.
The Dempster Highway in northern Canada is named for him.