Zwettl (German pronunciation: [ˈt͡svɛtl̩]; Central Bavarian: Zwedl; Czech: Světlá) is a town and district capital of the Austrian state of Lower Austria.

Although the etymology suggests an early population of Slavic people no archeological evidence has been found yet.

Today, the Cistercian convent in Zwettl houses the only remaining manuscript of the life of the beguine mystic Agnes Blannbekin.

The town is found in the middle of Waldviertel at the confluence of the Kamp and Zwettl rivers at the upper part of Kamptal.

After Vienna and Wolfsberg in Carinthia, it is the third largest municipality in Austria by area.