We have observed Stark shifts of quantum well intersubband transitions in a perpendicular electric field. Two samples consisting of 100 and 120 A˚ GaAs quantum wells separated by 350 A˚ AlGaAs barriers showed optical absorption peaks at 11.1 and 13.9 &mgr;m, respectively. In an electric field of 36 kV/cm, the 13.9 &mgr;m peak shifted to 13.7 &mgr;m and the 11.1 &mgr;m shifted to 11.0 &mgr;m, in good agreement with theoretical calculations. These tunable transitions can be applied to high‐speed infrared light modulators.