Vas (Hungarian: Vas vármegye, pronounced [ˈvɒʃ]; German: Komitat Eisenburg; Slovene: Železna županija or županija Železna; Croatian: Željezna županija) is an administrative county (comitatus or vármegye) of Hungary. It was also one of the counties of the former Kingdom of Hungary. It is part of the Centrope Project.

Vas County lies in western Hungary. It shares borders with Austria (Burgenland), Slovenia (Mura Statistical Region), and the Hungarian counties of Győr-Moson-Sopron, Veszprém, and Zala. The capital of Vas County is Szombathely. Its area is 3,336 km².

Vas is also the name of a historic administrative county (comitatus) of the Kingdom of Hungary. Its territory is now in western Hungary, eastern Austria, and eastern Slovenia. The capital of the county was Szombathely.

Vas County arose as one of the first comitatus of the Kingdom of Hungary.

In 1920, by the Treaty of Trianon 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 (later renamed to Yugoslavia). In the Vendvidék in 1919 was founded an unrecognized state the Prekmurje Republic, similar to Burgenland the Lajtabánság. The remainder stayed in Hungary, as the present Hungarian county Vas. After World War II due to Communist reforms, a small part of the 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 Yugoslav part of the former Vas County (around Murska Sobota) is part of Slovenia.

Vas County is home to a small Slovene minority, which lives in the area between the town of Szentgotthárd and the Slovenian border (see Hungarian Slovenes).

In 2015, the county had a population of 253,997 and the population density was 76/km².