John Francis Xavier Diffley (born 4 March 1958) is an American biochemist and Associate Research Director at the Francis Crick Institute.
He then worked as a postdoctoral researcher with Bruce Stillman at Cold Spring Harbor Laboratory.
His group studies the mechanism and regulation of eukaryotic DNA replication.
In 1998, Diffley was elected a member of European Molecular Biology Organisation (EMBO).
He is also an elected member of Academia Europaea (2009) and the European Academy of Cancer Sciences (2011).