University of Giessen, official name Justus Liebig University Giessen (German: Justus-Liebig-Universität Gießen), is a large public research university in Giessen, Hesse, Germany. It is named after its most famous faculty member, Justus von Liebig, the founder of modern agricultural chemistry and inventor of artificial fertiliser. It covers the areas of arts/humanities, business, dentistry, economics, law, medicine, science, social sciences, and veterinary medicine. Its university hospital, which has two sites, Giessen and Marburg (the latter of which is the teaching hospital of the University of Marburg), is the only private university hospital in Germany.

The University of Giessen is among the oldest institutions of higher educations in the German-speaking world. It was founded in 1607 as a Lutheran university in the city of Giessen in Hesse-Darmstadt because the all-Hessian Landesuniversität (the nearby University of Marburg (Philipps-Universität Marburg) in Marburg, Hesse-Kassel (or Hesse-Cassel)) had become Reformed (that is, Calvinist). Louis V, Landgrave of Hesse-Darmstadt, whence the university got its original name "Ludoviciana," founded his own institution of higher education in Giessen, which as a Lutheran institution had the primary function of ensuring the education of pastors and civil servants. Endowed with a charter issued by Rudolf II, Holy Roman Emperor, on 19 May 1607, the university was allowed to proceed with instruction in October 1607. During the Thirty Years' War, when Hesse-Darmstadt was able to take the area around Marburg for itself, the University of Giessen ceased instruction and was moved back to its more long-standing location in Marburg (1624/25). The Peace of Westphalia led to the restoration of the old location and in 1650 to the relocation of the university to Giessen.

In the 17th and 18th centuries the Ludoviciana was a typical small state university that then had the four common faculties (theology, law, medicine, and philosophy). The instruction was reasonable, with about 20 to 25 professors teaching several hundred students, the latter of which were mostly "Landeskinder." In the 18th century came gradual modernization of the curricula and reforms in the instruction, which were definitively influenced by the local lordly court in Darmstadt. The example for the reforms were both of the "model universities of the Enlightenment," the University of Halle, founded in 1694, and more still Georgia Augusta, founded in Göttingen in 1734/37. Indeed, all attempts at reform were from the start limited by the limited finances of Hesse-Darmstadt.