Robert Marcus Gunn (1850, Dunnet – 29 November 1909, Hindhead) was a Scottish ophthalmologist remembered for Gunn's sign and the Marcus Gunn pupil.
Gunn went to school in Golspie, then studied medicine at the University of St Andrews and the University of Edinburgh, graduating with distinction M.A. in 1871 and M.B.,C.M. in 1873. He was influenced by James Syme, Joseph Lister and Douglas Argyll Robertson during his studies. He taught himself direct ophthalmoscopy, a skill for which he would later become particularly noted.
Gunn was a house physician at Moorfields Eye Hospital and then worked in comparative anatomy at University College Hospital. He worked at the Perth District Asylum during the summers of 1874 and 1875, where he examined the fundi of all the patients. He spent 6 months in Vienna working under Eduard Jäger von Jaxtthal. He returned to Moorfields in 1875, becoming junior house surgeon and then senior house surgeon in 1876. He introduced Lister's sterile technique, improving the results of cataract surgery.
In December 1879, Gunn travelled to Australia to collect eye specimens from indigenous animals, publishing work on comparative anatomy of the eye in the Journal of Anatomy and Physiology. He also examined specimens from the Challenger expedition after his return to England.