Transcranial Alternating Current Stimulation (tACS) is a novel non-invasive brain stimulation modality. tACS passes weak, oscillating electric currents through the scalp to generate electric fields in the cerebral cortex. tACS has been shown to modulate both cortical oscillations and cognitive function in humans. These findings suggest that tACS is a promising neurotherapeutic for the treatment of psychiatric illnesses with aberrant underlying cortical dynamics such as schizophrenia. Due to the complexity of the network dynamics in brains of patients with neuropsychiatric disorders, it is unlikely that the feedforward application of tACS without neuronal feedback will be sufficient to effectively control the pathological network dynamics. To our knowledge, no successful EEG-based feedback brain stimulation with tACS has been reported to date. We here present an EEG feedback-controlled tACS system that controls cortical state dynamics. We used occipitoparietal stimulation in the gamma frequency band (40 Hz) in response to the endogenous alpha frequency activity in the visual cortex. We found that EEG feedback-controlled 40 Hz tACS (FB tACS) was more effective at controlling the amplitude of alpha-band cortical oscillations than dose-matched randomly-administered 40 Hz tACS (RA tACS). Our novel system therefore represents an important step towards the development of individualized therapeutic brain stimulation that is based on feedback control.