Charles-Eugène Guye (French pronunciation: [ʃaʁl øʒɛn ɡi]; October 15, 1866 – July 15, 1942) was a Swiss experimental physicist noted for his experiments showing the dependence of the electron mass on its speed and attending the Solvay Conferences.

Starting in 1907 and continuing for over a decade, he and his students Simon Ratnowsky and Charles Lavanchy conducted experiments with cathode rays that demonstrated the dependence of the electron mass on its speed,[6][7] with results supporting the predictions of Lorentz, Einstein, and the special theory of relativity against Max Abraham's rival theory of the electron.

Furthermore, he was editor-in-chief of Archives des Sciences physiques et naturelles and member of the editorial board of Helvetica Physica Acta.

[4] He participated in the 5th (Leiben, 1927) and 7th (Paris, 1933) Solvay Conferences,[8] and was the author or co-author of over 200 papers in physics and several popular books, including philosophical works on the biological-physical-chemical basis of evolution and the limits of physics and biology.

[4][9] In 2017 the European Physical Society declared the Bastions building of the Geneva University as an EPS Historic Site in honor of the scientific achievements made by Guye and Ernst Stueckelberg in this building.