The Copenhagen Fire of 1728 was the largest fire in the history of Copenhagen, Denmark. It began on the evening of 20 October 1728 and continued to burn until the morning of the 23rd of October 1728. It destroyed approximately 28% of the city (measured by counting the number of destroyed lots from the cadastre) and left 20% of the population homeless. The reconstruction lasted until 1737. No less than 47% of the section of the city, which dates back to the Middle Ages, was completely lost, and along with the Copenhagen Fire of 1795, it is the main reason that few traces of medieval Copenhagen can be found in the modern city.
Although the number of dead and wounded was relatively low compared to the extent of the fire, the cultural losses were huge. In addition to several private book collections, 35,000 texts including a large number of unique works were lost with the University of Copenhagen library, and at the observatory on top of Rundetårn, instruments and records made by Tycho Brahe and Ole Rømer were destroyed.