Det Classenske Fideicommis (literally "The Classen Fideicommiss") is a Danish charitable foundation. By testament in 1789 and his codicil of March 23, 1792, the industrialist Major General Johan Frederik Classen left behind his wealth and possessions as a fund, among other things, to "alleviate poverty and misery". It grants about 2 million kroner ($340,000) annually.
Classen (1725–1792) was an enterprising businessman and industrialist who developed an armaments and munitions factory in northwest Zealand. He called it Frederiksværk (Frederik's factory) in honour of King Frederik V who had provided land and incentives for its establishment. Thanks to the profits he made as sole supplier to the Danish army, especially during the Seven Years' War (1756–1763), Classen was able to expand his production facilities and attract other industries to Frederiksværk which quickly became a thriving town and major industrial centre. In addition to his business activities, Classen was adept at farming and forestry. In 1768, he bought land on Falster, where he comprehensively renovated Corselitze, a large manor house, and in 1773 he developed large-scale plantations in the Frederiksværk area. Major General Classen built himself a summerhouse by the Corselitze Forest.
Classen had built up a library of some 20,000 volumes as well as a large collection of minerals, instruments, and sheet music. For many years he had suffered from gout which was particularly painful towards the end. By the time of his death, he had become one of the most important figures in the country with titles of major general and knight of the Dannebrog and the style of excellency.