Altheimer is a city in Plum Bayou Township, Jefferson County, Arkansas, United States. It is situated on the Union Pacific Railway, 11 miles (18 km) northeast of Pine Bluff. As of the 2010 census it had a population of 984, down from 1,192 at the 2000 census. As of 2018 the estimated population was 829 and was down to 696 in the 2020 Census, with zero change estimated in 2021 and 2022.