Sir John Bretland Farmer FRS (5 April 1865 – 26 January 1944) was a British botanist, born at Atherstone and educated at Magdalen College, Oxford. He was fellow of Magdalen in 1889–1897, demonstrator of botany in 1887–1892, and assistant professor of biology in 1892–1895 at Oxford, and then became professor of botany at Imperial College London. He received the Doctor of Science (D.Sc.) from the University of Oxford in March 1902.
Farmer was an editor of the Annals of Botany and wrote particularly on cytology.
He was elected a Fellow of the Royal Society in 1900, was awarded its Royal Medal in 1919 and was its vice-president from 1919 to 1921. He was knighted in 1926.