Gießen came into being as a waterside castle in 1152, built by Count Wilhelm von Gleiberg. But the history of the community in the northeast and in today's suburb called "Wieseck" dates back to 775. In 1607, the University of Gießen was founded. In 1977 Gießen was merged with the neighboring city Wetzlar to form the new city of Lahn; however, this attempt to reorganize the administration was reversed in 1979.
Heavy bombing destroyed about 75% of Gießen in 1944, including most of the city's historic buildings. An American military base was located in Gießen after World War II. The U.S. Army Garrison Gießen, has a population of 500 Americans. The base is a converted German Army Air Field, which is reflected in some of the buildings, including the housing area. A theatre known as the Keller Theatre is a converted German army Officer's Club.