Louis Prosper Gachard (12 March 1800 – 24 December 1885), Belgian man of letters, was born in Paris.
He entered the administration of the national archives in 1826, and was appointed director-general in 1831, a post which he held for fifty-five years.
It includes 246 entries.
He was the author of several historical writings, of which the best known are Don Carlos et Philippe II (1863), Études et notices historiques concernant l'histoire des Pays-Bas (1863), Histoire de la Belgique au commencement du XVIIIème siècle (1880), Histoire politique et diplomatique de P.