Marc Delafontaine (March 31, 1837/1838, Céligny, Switzerland[a]–1911[b]) was a Swiss chemist and spectroscopist who was involved in discovering and investigating some of the rare earth elements.

Delafontaine studied with Jean Charles Galissard de Marignac at the University of Geneva.

[1] Delafontaine moved to the United States of America, arriving in New York in 1870,[2] and later becoming a naturalized citizen.

[1] In 1843, Carl Gustaf Mosander discovered terbium and erbium as components of yttria.

[8] In 1864, Marc Delafontaine used optical spectroscopy to conclusively prove that yttrium, terbium, and erbium were separate elements.