William John Dundas, FRSE, LLD, WS (1849–1921) was a Scottish lawyer and mathematician.

He was born on 16 March 1849 at Ochtertyre House, Stirlingshire, the son of Elizabeth Mackenzie and George Dundas, Lord Manor.

His lawyer practice ran from 16 St Andrew Square in Edinburgh's First New Town.

His brother was the military hero Captain James Dundas VC.

His great nephew and namesake William John Dundas (1923–1965) was one of the three survivors on the sinking of HMS Hood by the Bismarck in 1941.