John Johnstone (1768–1836) was a British physician and biographer.
He studied at Kidderminster Free School and entered Merton College, Oxford, in 1786.
He practised medicine in Worcester from 1793 to 1799, when he moved to Birmingham, where he built up a large practice.
He was president of the second meeting of the Provincial Medical and Surgical Association in 1834.
They had two daughters, Anna Delicia (born 1811), who married Walter Farquhar Hook, and Agnes Mary (born 1814), who married the Rev.