Lars Onsager (November 27, 1903–October 5, 1976) was a Norwegian-born American physical chemist and theoretical physicist, winner of the 1968 Nobel Prize in Chemistry.

In 1925 he arrived at a correction to the Debye-Hückel theory of electrolytic solutions, to take care of Brownian movement of ions in solution. He so thoroughly impressed Debye that he was invited to become Debye's assistant at the Eidgenössische Technische Hochschule (ETH), where he remained until 1928.
In 1933, when the Great Depression limited Brown's ability to support a faculty member who was only useful as a researcher and not a teacher, he was let go by Brown, being hired after a trip to Europe by Yale University, where he remained for most of the rest of his life, retiring in 1972.
In 1945, Onsager was naturalized as an American citizen, and the same year he was awarded the title of J. Willard Gibbs Professor of Theoretical Chemistry.
In 1947, he was elected to the National Academy of Sciences, and in 1950 he joined the ranks of Alpha Chi Sigma.
He developed important ideas on the quantization of magnetic flux in metals. He was awarded the Lorentz Medal in 1958 and the Nobel Prize in Chemistry in 1968.