We have achieved single‐mode operation in a pulsed hybrid optically pumped D2O far‐infrared laser. The active volume of the resonator was divided into two sections separated by a thin plastic foil. The larger section served as the main gain medium and the shorter section as the mode selective element. The vapor pressure in the smaller volume was either very low or alternatively about three times higher than the pressure in the main part. In both cases single‐mode operation was achieved without any reduction of the total output energy.