Ahmed Hassan Zewail (February 26, 1946 – August 2, 2016) was an Egyptian American chemist, known as the "father of femtochemistry". He was awarded the 1999 Nobel Prize in Chemistry for his work on femtochemistry and became the first Egyptian to win a Nobel Prize in a scientific field, and the second African to win a Nobel Prize in Chemistry. He was the Linus Pauling Chair Professor of Chemistry, Professor of Physics, and the director of the Physical Biology Center for Ultrafast Science and Technology at the California Institute of Technology.

Ahmed Hasan Zewail was born on February 26, 1946, in Damanhur, Egypt, and was raised in Desouk. He received a Bachelor of Science and Master of Science degrees in Chemistry from Alexandria University before moving to the United States to complete his PhD at the University of Pennsylvania supervised by Robin M. Hochstrasser.

After completing his PhD, Zewail did postdoctoral research at the University of California, Berkeley, supervised by Charles Bonner Harris. Following this, he was awarded a faculty appointment at the California Institute of Technology in 1976, and he made the first Linus Pauling Chair in Chemical Physics. He became a naturalized citizen of the United States on 5 March 1982. Zewail was the director of the Physical Biology Center for Ultrafast Science and Technology at the California Institute of Technology.

Zewail had been nominated and participated in President Barack Obama's Presidential Council of Advisors on Science and Technology (PCAST), an advisory group of the nation's leading scientists and engineers to advise the President and Vice President and formulate policy in the areas of science, technology, and innovation.

Zewail's key work was a pioneer of femtochemistry—i.e. the study of chemical reactions across femtoseconds. Using a rapid ultrafast laser technique (consisting of ultrashort laser flashes), the technique allows the description of reactions on very short time scales – short enough to analyse transition states in selected chemical reactions. Zewail became known as the "father of femtochemistry".