Lifetime of aluminum electrolytic capacitors is of paramount importance, since in many power electronic voltage source converter systems this capacitor type is used in the DClink. Thermal stress is one of the most critical stressors for electrolytic capacitors leading to wear-out and failures. Therefore, methods to perform online monitoring of the hotspot temperature of capacitors would lead to beneficial information, which could be used for protection or lifetime prediction purposes. This paper proposes a hotspot temperature estimation method based on the linear dependence between capacitance and temperature of electrolytic capacitors. For this, a set of nine samples of the same aluminum capacitor type is characterized by means of capacitance measurements at different hotspot temperatures. The obtained linear correlation is used to calibrate hotspot temperature estimation for the investigated electrolytic capacitor type. It is verified experimentally that the temperature estimation error of the proposed method is well below 5 K. Thus, the proposed procedure can be used as a basis for future research on improving condition monitoring of electrolytic capacitors.