Robert Sanderson Mulliken (June 7, 1896–October 31, 1986) was an American physicist and chemist, primarily responsible for the early development of molecular orbital theory, i.e. the elaboration of the molecular orbital method of computing the structure of molecules. Dr. Mulliken received the Nobel Prize for chemistry in 1966. He received the Priestley Medal in 1983.

He graduated in 1913 and succeeded in getting a scholarship to MIT which had earlier been won by his father. Like his father, he majored in chemistry. He received his B. S. degree in chemistry from MIT in 1917.
He got his doctorate in 1921 based on research into the separation of isotopes of mercury by evaporation, and continued in his isotope separation by this method.
In 1925 and 1927, Mulliken traveled to Europe, working with outstanding spectroscopists and quantum theorists such as Erwin Schrödinger, Paul A. M. Dirac, Werner Heisenberg, Louis de Broglie, Max Born, and Walther Bothe (all of whom eventually received Nobel Prizes) and Friedrich Hund, who was at the time Born's assistant.
In 1927 Mulliken worked with Hund and as a result developed his molecular orbital theory, in which electrons are assigned to states that extend over an entire molecule.
From 1926 to 1928, he taught in the physics department at New York University (NYU). This was his first recognition as a physicist; though his work had been considered important by chemists, it clearly was on the borderline between the two sciences and both would claim him from this point on.
In 1934, he derived a new scale for measuring the electronegativity of elements. This does not entirely correlate with the scale of Linus Pauling, but is generally in close correspondence.
In World War II, from 1942 to 1945, Mulliken directed the Information Office for the University of Chicago's Plutonium project. Afterward, he developed mathematical formulas to enable the progress of the molecular-orbital theory.
In 1952 he began to apply quantum mechanics to the analysis of the reaction between Lewis acid and base molecules. (See Acid-base reaction theories.) He became Distinguished Professor of Physics and Chemistry in 1961 and continued in his studies of molecular structure and spectra, ranging from diatomic molecules to large complex aggregates. He retired in 1985.
He died of congestive heart failure at his daughter's home in Arlington, Virginia (across the Potomac River from Washington, D.C.) His body was returned to Chicago for burial.