Hermiswil is a former municipality in the Oberaargau administrative district in the canton of Bern in Switzerland.

In 1466 the city of Solothurn inherited all the vom Stein holdings, which included Hermiswil.

It was placed under the authority for low justice from Bollodingen and administered by the Landvogtei of Wangen.

The Roman Catholic citizens of Hermiswil were placed under the guidance of the Reformed parish of Herzogenbuchsee.

For the rest of the land, 1.18 km2 (0.46 sq mi) or 5.1% is settled (buildings or roads), 0.04 km2 (9.9 acres) or 0.2% is either rivers or lakes.

32.6% of the total land area is heavily forested and 1.2% is covered with orchards or small clusters of trees.