Professor Sir John Ernest Walker (born January 7, 1941) is an English chemist who won the Nobel Prize in Chemistry in 1997.

He received a B.A. degree from St Catherine's College, Oxford.
He began study of peptide antibiotics with Edward Abraham at Oxford in 1965 and received his DPhil in 1969.
From 1969 to 1971, he worked at the University of Wisconsin–Madison, and from 1971–1974 in France. He met Fred Sanger in 1974 at a workshop at Cambridge University.
At first, he analyzed the sequences of proteins and then uncovered details of the modified genetic code in mitochondria. In 1978, he decided to apply protein chemical methods to membrane proteins.
Sir John was knighted in 1999 for services to molecular biology.