Objective Recent developments in transcranial alternating current stimulation (tACS) provide a powerful approach to establish the functional roles of neuronal oscillatory activities in the human brain. Here, we investigated whether tACS can reach and modulate the excitability of the visual cortex in a frequency-dependent manner. Methods We measured the cortical excitability of the visual cortex using single pulse transcranial magnetic stimulation (TMS) while delivering tACS to the occipital region at different frequencies (5, 10, 20 and 40Hz). Results We found that tACS at 20Hz decreased TMS–phosphene threshold (i.e., increased the excitability of the visual cortex) during the stimulation, whereas other frequencies did not affect TMS–phosphene thresholds. Conclusions Our findings demonstrate direct interactions of tACS with the visual cortex in a frequency-dependent manner. Significance Our present work provides further demonstration of the potential of tACS as a method to selectively modulate the excitability of the visual cortex.