Untersteckholz was a municipality in the district of Aarwangen in the Canton of Bern in Switzerland . On 1 January 2010, Untersteckholz merged into Langenthal.

Before the merger, Untersteckholz had an area, as of 2009, of 2.83 km2 (1.09 sq mi). This area uses 2.09 km2 (0.81 sq mi) or 73.9% for agricultural purposes, while 0.59 km2 (0.23 sq mi) or 20.8% is forested. Of the remaining land, 0.16 km2 (0.062 sq mi) or 5.7% is settled (buildings or roads).

Housing and buildings comprised 2.1% of the built-up area, and transportation infrastructure comprised 3.2%.