Vas (German: Eisenburg, Slovene: Železna županija) is the name of an administrative county (comitatus or megye) in present Hungary, and also in the former Kingdom of Hungary. The county is a part of the Centrope Project.
Vas county lies in western Hungary. It shares borders with Austria and Slovenia and the Hungarian counties Győr-Moson-Sopron, Veszprém and Zala. The capital of Vas county is Szombathely. Its area is 3,336 km².
In 1918 (confirmed by the Treaty of Trianon 1920), the western part of the county became part of the new Austrian land Burgenland, and a smaller part in the southwest, known as Vendvidék became part of the newly formed Kingdom of Serbs, Croats and Slovenes (Yugoslavia). In the Vendvidék in 1919 was founded an unrecognized state the Prekmurje Republic, alike in Burgenlad the Lajtabánság. The remainder stayed in Hungary, as the present Hungarian county Vas. A small part of former Sopron county went to Vas county. Some villages north of Zalaegerszeg went to Zala county, and a small region west of Pápa went to Veszprém county.
Since 1991, when Slovenia became independent from Yugoslavia, the Yugoslavian part of former Vas county (around Murska Sobota) is part of Slovenia.
The Vas county is home to a small Slovene minority, which lives in the area between the town of Szentgotthard and the Slovenian border (see Hungarian Slovenes).