We report the existence of a low-frequency oscillation of 10 mHz in the ionosphere prior to the substorm onset in three substorm events. A substorm is an explosive event in which the stored magnetic energy in the Earth's magnetotail is violently released leading to geomagnetic disturbances that can be observed in the magnetosphere and ionosphere. The 10 mHz wave power starts increasing 2-5 min prior to substorm onset and peaks around the onset. The 10 mHz wave power starts declining about the time when electron energization is suddenly observed, which may suggest a causal relationship. The analysis of the bead structures in the auroral images suggest that the origin of this low-frequency oscillation is most likely related to a type of plasma instability in the magnetotail known as kinetic ballooning instability. A few min before the onset, during the substorm growth phase, the magnetotail condition is favorable for a kinetic ballooning instability leading to the growth of the 10 mHz wave. The low frequency oscillation of 10 mHz in the ionosphere can be taken as a preonset signature of the substorm onset with a precursor of several min or longer.