William Pitcairn (9 May 1712 - 25 November 1791) was a Scottish physician and botanist.

He was born in Dysart, Fife, the second son of the Revd David Pitcairn. A younger brother was John Pitcairn, who became a British Marine officer and was killed at the Battle of Bunker Hill; John's son and William's nephew was midshipman Robert Pitcairn, for whom Pitcairn Island