A new type of instability of standing gravity—capillary waves has been detected under the irradiation of the liquid surface by a continuous plane ultrasonic beam with a constant amplitude. At an intensity of 1.03-MHz ultrasound above a certain threshold, the excitation of one of the eigenmodes of gravity—capillary waves at a frequency of 6.56 Hz has been observed. The time of establishment of the stationary amplitude of oscillations of the surface significantly exceeds the intrinsic time of relaxation of the mode and decreases with an increase in the threshold excess. The scenario of development of instability is similar to an inverse energy cascade.