Transdanubia (Hungarian: Dunántúl [ˈdunaːntuːl]; German: Transdanubien, Croatian: Prekodunavlje[1] or Zadunavlje, Slovak: Zadunajsko[2]) is a traditional region of Hungary.

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.

Transdanubia is essentially a Hungarian geographical concept so these areas ceased to be parts of it when they were annexed by neighbouring countries.

Historically the counties of Transdanubia were Moson, Győr, Sopron, Vas, Zala, Veszprém, Fejér, Komárom, Esztergom, Somogy, Tolna and Baranya.

With some present-day Austrian and Croatian territories, it comprised the Province of Pannonia, a romanised, Latin-speaking border region with important Roman towns (Scarbantia, Aquincum, Sopianae, Gorsium, Savaria) and rural villas.

In the Age of Migrations it was occupied by the Huns, Ostrogoths, Lombards, Gepids, Avars, Franks and the Slavic peoples.

Esztergom has been the ecclesiastical capital of the country since 1001 until today, and Stephen I of Hungary established his royal seat at Székesfehérvár, later the coronation town of the kings.

The Asian-style Ottoman rule and the constant war had devastating effects: dozens of villages were destroyed, the population was decimated and parts of the region became almost totally deserted.

In the 18th century the region was slowly rebuilt, and many new settlers (Magyars, Germans, Poles, Slovaks, and Croatians) arrived.

Famous baroque castles were built to show the power of the big landowner families, for example the Esterházys in Fertőd and the Festetics in Keszthely.