A double‐prism variable‐beam attenuator for 10.6 &mgr;m, which is based on frustrated total internal reflection, has been built. The attenuation can be varied between 2 dB and more than 105 dB, and the transmitted power is independent of the beam polarization. For high attenuations, the phase behavior is that of a quarter‐wave plate. With the aid of a visible laser beam, the prism gap can be monitored interferometrically.