English-born American plant physiologist who isolated auxin, an important plant growth hormone.

Thimann studied chemistry at Imperial College in London, where he received a Ph.D. in biochemistry in 1928. After teaching for two years at King's College for Women in London, Thimann went to the United States, where he served on the faculties at the California Institute of Technology in Pasadena (1930–35), Harvard University (1935–65), and the University of California at Santa Cruz from 1965. He became a U.S. citizen in 1941.

Thimann began his search for the growth hormone with Hermann Dolk in the early 1930s at the California Institute of Technology. Most of the work was completed when Dolk died in 1933; the next year Thimann obtained and isolated pure auxin in the form of β-indolylacetic acid (IAA). With several coworkers, Thimann proved that auxin promotes cell elongation, formation of roots, and growth of buds. These discoveries led to the development of a widely used synthetic auxin, 2,4-D. By using this and similar chemicals, the premature falling of fruit can be prevented and cut stems can be stimulated to grow abundant roots; in addition, because high concentrations of auxins are toxic to most plants, synthetic auxins are effective weed killers.

Thimann also demonstrated that the action of auxin on bud formation involves an interaction with another growth hormone, kinetin, isolated by Folke Skoog and Carlos Miller around 1956. Publications of his research include The Natural Plant Hormones (1972), Hormones in Living Plants (1977), and Senescence in Plants (1980).