Herisau is a municipality of the canton of Appenzell Ausserrhoden in Switzerland. It is the seat of the canton's government and parliament; the judicial authorities are situated in Trogen.

The central hamlet and the houses around the central square, the Protestant church of 1580, the houses Wetter and zur Rose (both 1737), the hamlet Schwänberg and the government building with the state archive are listed as heritage sites of national significance.

Herisau was first mentioned in 837 as Herinisauva, and its church is mentioned in 907. In 1084 Herisau was destroyed as part of battles around the monastery in St. Gallen. In 1248 and 1249 the town was destroyed again, this time by the monastery to establish loyalty. In 1401 Herisau joined an alliance with other places in Appenzell as part of the Appenzell Wars.

Between 1517 and 1518 Herisau managed to buy itself free from the monastery. The town hall was built in 1601. In 1606 the town was largely destroyed by a fire. In 1648 Schwellbrunn separates and becomes an independent village. Between 1798 and 1803 Herisau was the capital of the canton Säntis.