Transdanubia (Hungarian: Dunántúl; German: Transdanubien, Croatian: Prekodunavlje or Zadunavlje, Slovak: Zadunajsko) is a traditional region of Hungary.
Transdanubia can also refer to the 21st and 22nd districts of Vienna, which are the only ones lying on the left bank (eastern side), of the Danube. See Floridsdorf and Donaustadt respectively.
The borders of Transdanubia are the Danube River (north and east), the Drava and Mura rivers (south), and the foothills of the Alps roughly along the border between Hungary and Austria (west).
Transdanubia comprises the counties of Győr-Moson-Sopron, Komárom-Esztergom, Fejér, Veszprém, Vas, Zala, Somogy, Tolna, Baranya and the part of Pest that lies west of the Danube. (In the early Middle Ages the latter was known as Pilis county.)
This article deals with Transdanubia in this geographical meaning.
While the northern, eastern and southern borders of the region are clearly marked by the Danube and Drava rivers, the western border was always identical with the political boundary of Hungary, therefore it was affected by the territorial changes of the 20th century. Before the Treaty of Trianon in 1920 the present-day regions of Burgenland, Prekmurje and Međimurje were integral parts of Transdanubia. The three villages of Rusovce, Jarovce and Čunovo also belonged to Transdanubia before the Paris Peace Treaty in 1947. Transdanubia is essentially a Hungarian geographical concept so these areas ceased to be parts of it when they were annexed by neighbouring countries.