Francis William Aston (born Birmingham, September 1, 1877; died Cambridge, November 20, 1945) was a British physicist who won the 1922 Nobel Prize in Chemistry for the invention of the mass spectrometer.

In 1903 he won a scholarship to the University of Birmingham and it was in his studies of electronic discharge tubes there that he discovered the phenomenon now known as the Aston Dark Space. In 1909 he moved to the Cavendish Laboratory in Cambridge on the invitation of J.J. Thomson and worked on the identification of isotopes of the element neon. Returning to these studies after the First World War in 1919, he used a method of electromagnetic focusing to invent the mass spectrograph, which rapidly allowed him to identify no fewer than 212 of the 287 naturally occurring isotopes.

His work on isotopes also led to his formulation of the Whole Number Rule which states that "the mass of the oxygen isotope being defined, all the other isotopes have masses that are very nearly whole numbers," a rule that was used extensively in the development of nuclear energy.