About 10 years ago, Potsdam started with an ambitious program to reduce its climate impact, including the substitution of fossil energy sources for district heating by renewable energies. The subsurface exploration of the geothermal potential of the city area forms an important and challenging component of this strategy. Several deep saline aquifers in the area of Potsdam show potential for geothermal utilization, from which the Aalenian sandstone (Middle Jurassic) has been selected for further exploitation. Here, we report on the exploration results, the doublet drilling, the formation testing, and the scientific evaluation of the Aalenian geothermal reservoir sandstone. The outcome is discussed with respect to its future use in the district heating system of the surrounding residential buildings and to its relevance for the extended geothermal field development in Potsdam.