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. During this long period he reorganized the service, added to the records by copies taken in other European collections, travelled for purposes of study, and carried on a wide correspondence with other keepers of records, and with historical scholars.^[1]

He also edited and published many valuable collections of state papers; a full list of his various publications was printed in the Annuaire de l'Académie royale de Belgique by Ch. Piot in 1888, pp. 220–236. It includes 246 entries.^[1]

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. P. Rubens (1877), all published at Brussels.^[1]

His chief editorial works are the Actes des États généraux des Pays-Bas 1576-1585 (Brussels, 1861–1866), Collection de documens inédits concernant l'histoire de la Belgique (Brussels, 1833 1835), and the Relations des ambassadeurs vénitiens sur Charles V et Philippe II (Brussels, 1855). Gachard died in Brussels on 24 December 1885.^[1]