German organic chemist, winner of the Nobel Prize for Chemistry in 1928 for research on substances, notably vitamin D, that play important biological roles.

Windaus switched from medical to chemical studies. After receiving his Ph.D. from the University of Freiburg (1899), he held positions there and at Innsbruck, Austria, before his appointment as head of the chemical institute at the University of Göttingen (1915–44). His studies of the chemical structure of cholesterol, begun in 1901, spanned some 30 years. This work was part of his study of the complex alcohols known as sterols.

Windaus discovered 7-dehydrocholesterol, which is the chemical precursor of vitamin D, and he showed that it is a steroid. He discovered that it is converted into the vitamin when one of its chemical bonds is broken by the action of sunlight. This explained why exposure to sunlight can prevent vitamin D deficiency (rickets) in humans. Windaus' research also helped establish the chemistry of the sex hormones and advanced the development of drugs used to treat heart ailments.