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 merge, Untersteckholz had an area, as of 2009, of 2.83 km (1.09 sq mi). Of this area, 2.09 km (0.81 sq mi) or 73.9% is used for agricultural purposes, while 0.59 km (0.23 sq mi) or 20.8% is forested. Of the rest of the land, 0.16 km (0.062 sq mi) or 5.7% is settled (buildings or roads).

Of the built up area, housing and buildings made up 2.1% and transportation infrastructure made up 3.2%. 17.7% of the total land area is heavily forested and 3.2% is covered with orchards or small clusters of trees. Of the agricultural land, 56.5% is used for growing crops and 14.1% is pastures, while 3.2% is used for orchards or vine crops.

Untersteckholz has a population (as of 31 December 2020) of 166. As of 2007, 1.2% of the population was made up of foreign nationals. Over the last 10 years the population has grown at a rate of 5.8%. Most of the population (as of 2000) speaks German (97.6%), with Rhaeto-romance being second most common ( 0.6%) and Russian being third ( 0.6%).

In the 2007 election the most popular party was the SVP which received 65.3% of the vote. The next three most popular parties were the FDP (11.3%), the Green Party (11.1%) and the local small left-wing parties (5.9%).

The age distribution of the population (as of 2000) is children and teenagers (0–19 years old) make up 18.6% of the population, while adults (20–64 years old) make up 61.7% and the seniors (over 64 years old) make up 19.8%. In Untersteckholz about 79.7% of the population (between age 25–64) have completed either non-mandatory upper secondary education or additional higher education (either University or a Fachhochschule).

Untersteckholz has an unemployment rate of 0.17%. As of 2005, there were 58 people employed in the primary economic sector and about 14 businesses involved in this sector. 16 people are employed in the secondary sector and there are 5 businesses in this sector. 11 people are employed in the tertiary sector, with 2 businesses in this sector.