The winter of 1783/84 is known to have been severe and long-lasting in a number of European countries. Two very cold spells occurred: at the end of December 1783 and in January 1784. Furthermore, it snowed heavily in the months of December 1783, January and February 1784. On 21 February 1784, a warm southerly wind led to a thaw which resulted in fast breaking-up of the ice on the frozen rivers and to catastrophic floods. This large-scale and long-lasting event took place in the present-day Belgium, The Netherlands, Luxemburg, northern France, Germany, Austria, and the Czech and Slovak Republics. The above-mentioned event is studied for Belgium and for adjacent areas of its hydrological river basins. Emphasis is given to the hydrological phenomena, but these are, of course, strongly linked to the weather of that particular winter. Therefore, instrumental and non-instrumental climatological observations are presented and their relation to the floods is given. The main narrative data consist of two meteorological manuscripts of the Library of the Royal Observatory of Belgium that have never been used heretofore. The instrumental meteorological observations of the Mannheim Ephemerides series at Brussels, the Godart series at Verviers and Baron de Poederlé’s observations at Brussels are used. These narrative and instrumental meteorological data are further documented by quotations from a large number of contemporaneous authors and newspapers.