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Changes in pupil size at constant light levels reflect the activity of neuromodulatory brainstem centers that control global brain state. These endogenously driven pupil dynamics can be synchronized with cognitive acts. For example, the pupil dilates during the spontaneous switches of perception of a constant sensory input in bistable perceptual illusions. It is unknown whether this pupil dilation only indicates the occurrence of perceptual switches, or also their content. Here, we measured pupil diameter in human subjects reporting the subjective disappearance and re‐appearance of a physically constant visual target surrounded by a moving pattern (‘motion‐induced blindness’ illusion). We show that the pupil dilates during the perceptual switches in the illusion and a stimulus‐evoked ‘replay’ of that illusion. Critically, the switch‐related pupil dilation encodes perceptual content, with larger amplitude for …