Large Stark shifts of intersubband transitions in a step quantum well are observed for the first time. The Stark shifts are ∼8 and 7 meV at ∼18 kV/cm for the 1→2 and 1→3 intersubband transitions, respectively, while the Stark shift of a similar transition in a square quantum well is only about 0.5 meV under the same bias condition. The intersubband transitions in step quantum wells can be either red or blue Stark shifted depending on the direction of the applied electric field. The large Stark shifts of intersubband transitions in the step quantum wells can be exploited for the fabrication of optical modulators operating in the range from mid to far infrared.