Bad Buchau () (formerly Buchau) is a small town in the district of Biberach, Baden-Württemberg, Germany with about 4,000 inhabitants. It is situated near Lake Federsee, which is separated from the town by a wide reed belt. Bad Buchau incorporates the nine villages of Allmannsweiler, Dürnau, Kanzach, Betzenweiler, Moosburg, Alleshausen, Seekirch, Tiefenbach, and Oggelshausen, as well as the outlying farm settlements of Ottobeurer Hof, Bruckhof, and Henauhof. Also part of Bad Buchau is the formerly independent district of Kappel. The official language is German, with day-to-day conversations by the majority of its inhabitants in the Swabian dialect. From the 13th century to the mediatisation of 1803, Buchau had the particularity of being the seat of both an Imperial Abbey and a Free Imperial City. In terms of area, it was one of the smallest such cities and its island situation eliminated the necessity to erect city walls and towers. Buchau, however, lost its insular benefits after the water level of Lake Federsee had been lowered on two occasions.