Inner speech is one of the most common, but least investigated, mental activities humans perform. It is an internal copy of one’s external voice and so is similar to a well-established component of motor control: corollary discharge. Corollary discharge is a prediction of the sound of one’s voice generated by the motor system. This prediction is normally used to filter self-caused sounds from perception, which segregates them from externally caused sounds and prevents the sensory confusion that would otherwise result. The similarity between inner speech and corollary discharge motivates the theory, tested here, that corollary discharge provides the sensory content of inner speech. The results reported here show that inner speech attenuates the impact of external sounds. This attenuation was measured using a context effect (an influence of contextual speech sounds on the perception of subsequent speech sounds), which weakens in the presence of speech imagery that matches the context sound. Results from a control experiment demonstrated this weakening in external speech as well. Such sensory attenuation is a hallmark of corollary discharge.