Otto Paul Hermann Diels (January 23 1876 - March 7 1954), a German chemist. In 1950 he was awarded (with Kurt Alder) the Nobel Prize in Chemistry "for their discovery and development of the diene synthesis".

Diels taught until 1916 at the University of Berlin and from 1916 till 1945 at the University of Kiel. Two of his sons were killed in World War II.
In 1950 he and his student, Kurt Alder, were awarded the Nobel Prize in Chemistry "for their discovery and development of the cycloaddition synthesis". This organic synthesis is known also as the Diels-Alder reaction. It regioselectively produces up to four chiral centers and is one of the most useful reactions of its type.