Sir Isaac Bayley Balfour, KBE, FRS, FRSE (31 March 1853 – 30 November 1922) was a Scottish botanist. He was Regius Professor of Botany at the University of Glasgow from 1879 to 1885, Sherardian Professor of Botany at the University of Oxford from 1884 to 1888, and Professor of Botany at the University of Edinburgh from 1888 to 1922.

He was the son of John Hutton Balfour, also a botanist, and Marion Spottiswood Bayley, and was born at home, 27 Inverleith Row, Edinburgh. His mother was granddaughter of George Husband Baird.

He was the cousin of Sir James Crichton-Browne.

Balfour was educated at the Edinburgh Academy from 1864 to 1870. At this early stage his interests and abilities were in the biological sciences, which were taught to him by his father. Due to his father's post as Professor of Botany at Edinburgh, the young Balfour was able to visit the Edinburgh Botanical Gardens, not open to the public at the time.

Balfour studied at the University of Edinburgh, from which he graduated in 1873 with a BSc (1st Class) in Natural Science. In 1874 he participated in an astronomical expedition of 1874 to Rodrigues. Though the stated aim of the mission was to observe Venus, Balfour used the opportunity to investigate the local flora, and on his return, the fieldwork he had carried out permitted him to gain his doctorate in botany in 1875. Concurrent medical studies, including working as a wound dresser at Edinburgh Royal Infirmary under Joseph Lister the famous antisepsis pioneer, led him to graduate as a medical doctor with honours in 1877.

Between 1877 and 1879 Balfour also spent time at the universities in Würzburg and Strasbourg with the influential plant physiologist Julius von Sachs and plant pathologist Heinrich Anton de Bary. He later went on to translate some of their textbooks into English for The Clarendon Press (now Oxford University Press).

In 1879, his father resigned the chair at Edinburgh, Glasgow professor Alexander Dickson (1836–1887) was appointed in his place, and the younger Balfour was promoted to the chair of Regius Professor of Botany, Glasgow in Glasgow from 1879 to 1885. He also went on to lead an expedition to Socotra in 1880. During his 1880 expedition to the Island of Socotra, Bayley Balfour would make a huge breakthrough into discovering the origins of the color dragon's blood. This occurred when he discovered the plant “Dracaena cinnabari on Socotra island.” After collecting some samples he sent them to be analysed by “Glasgow's Professor of Chemistry James Dobbie and G Henderson.”This was very important as it marked the first scientific investigation into the origins of the color known as Dragons Blood. “Of the 15 Dracaena resin samples that are housed within the EBC some of them have certain geographic origins, most are of D. cinnabari from this collection of samples are from Socotra Balfour himself.” This was important as it showed that certain colors can only be found in certain geographic regions.

In 1884, he was appointed Sherardian Professor of Botany at the University of Oxford. and in the same year married Agnes Boyd Balloch. It was about this time, inspired by the kind of experimental botany he experienced at Würzburg  and  Strasbourg, Balfour founded an English language journal for experimental and observational botany entitled 'Annals of Botany' in collaboration with other young botanists most notably Sydney Howard Vines. Appearing in 1887 and published by the Clarendon Press, Annals of Botany remains one of the world’s leading plant science journals