Trencsén county (Latin: comitatus Trentsiniensis / Trenchiniensis; Hungarian: Trencsén (vár)megye; Slovak: Trenčiansky komitát / Trenčianska stolica / Trenčianska župa; German: Trentschiner Gespanschaft / Komitat) was an administrative county (comitatus) of the Kingdom of Hungary.

A predecessor of the Trencsén county maybe existed already in the 9th century, at the time of Great Moravia, with a center in Ducové.

The Trencsén county as a Hungarian comitatus arose at the end of the 11th century, when most parts of the territory were conquered by the Kingdom of Hungary.

Traditionally, the office of hereditary lord lieutenant of Trencsén county was held by the Csák, Cseszneky and Illésházy families.

In the aftermath of World War I, Trencsén county became part of newly formed Czechoslovakia, as recognized by the concerned states in 1920 by the Treaty of Trianon.