The Classen Library (Danish: Det Classenske Bibliotek) was a public library in Copenhagen, Denmark, created from the private book collection of Johan Frederik Classen at the time of his death in 1792. It was the third largest library in the city, surpassed only by the Royal Danish Library and Copenhagen University Library and existed until 1867 when it was merged with the latter.Its building at 38 Amaliegade now houses Det Konserveringsfaglige Videncenter, a centre for conservation.