Lettre à M. Dacier (full title: Lettre à M. Dacier relative à l'alphabet des hiéroglyphes phonétiques: "Letter to M. Dacier concerning the alphabet of the phonetic hieroglyphs") is a scientific communication in the form of a letter sent in 1822 by the Egyptologist Jean-François Champollion to Bon-Joseph Dacier, secretary of the French Académie des Inscriptions et Belles-Lettres. It is the founding text upon which Ancient Egyptian hieroglyphics were first systematically deciphered by Champollion, largely on the basis of the multilingual Rosetta Stone.