Finnish-born Swedish physiologist who was a corecipient (with George Wald (Wald, George) and Haldan Hartline (Hartline, Haldan Keffer)) of the 1967 Nobel Prize for Physiology or Medicine for his analysis of the internal electrical changes that take place when the eye is exposed to light.

Granit received an M.D. degree from the University of Helsinki in 1927, after which he conducted research at the University of Pennsylvania and at the laboratory of Sir Charles Scott Sherrington at Oxford, England. He was appointed professor of physiology at the University of Helsinki in 1937. A naturalized Swede, Granit joined the medical school of the Karolinska Institute, Stockholm, in 1940; he was named chairman of the institute's department of neurophysiology in 1946. A year earlier he had also become the director of the Nobel Institute for Neurophysiology in Stockholm. In the 20 years from 1956 to 1976 Granit also served as a visiting professor or researcher at numerous institutions.

From studies of the action potentials in single fibres of the optic nerve, Granit formed his “dominator-modulator” theory of colour vision. In this theory he proposed that in addition to the three kinds of photosensitive cones—the colour receptors in the retina—which respond to different portions of the light spectrum, some optic nerve fibres (dominators) are sensitive to the whole spectrum while others (modulators) respond to a narrow band of light wavelengths and are thus colour-specific. Granit also proved that light could inhibit as well as stimulate impulses along the optic nerve. His book Sensory Mechanisms of the Retina (1947) is a classic work in the field of retinal electrophysiology.

Granit then turned his attention to the study of the control of movement, specifically the role of muscle sense-organs called muscle spindles and tendon organs. He helped to determine the neural pathways and processes by which these internal receptors regulate and coordinate muscle action.