Otto Paul Hermann Diels (January 23, 1876 – March 7, 1954) was a German chemist. He was the son of a professor of philology at the University of Berlin, where he himself earned his doctorate in chemistry, in the group of Emil Fischer.
Diels taught till 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 was awarded (with Kurt Alder, his student) the Nobel Prize in Chemistry "for their discovery and development of the diene synthesis," known also as the Diels-Alder Reaction. This amazing reaction regioselectively produces up to four chiral centers and is widely considered to be one of the most powerful reactions in organic synthesis.