Louis Prosper Gachard (March 12, 1800 - December 24, 1885), Belgian man of letters, was born in Paris.

He entered the administration of the royal archives in 1826, and was appointed director-general, 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.

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.

He was the author of several historical writings, of which the best known are Don Carlos et Philippe II (1867), Etudes et notices historiques concernant l'histoire des Pays-Bas (1863), Histoire de la Belgique au commencement du XIXieme siècle (1880), Histoire politique et diplomatique de P. P. Rubens (1877), all published at Brussels.

His chief editorial works are the Actes des tlrts généraux des Pays-Bas 1576-1585 (Brussels, 1861-1866), Collection de documents inedits concernant l'histoirede la Belgique (Brussels, 1833 1835), and the Relations des ambassadeurs Vénitiens sur Charles V et Philippe II (Brussels, 1855). Gachard died in Brussels on the 24th of December 1885.