Noninvasive cortical stimulation techniques, such as transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS), have proved to be powerful tools for establishing causal relationships between brain regions and their functions. In the present study, we demonstrate that a new technique called transcranial alternating current stimulation (tACS) can interact with ongoing rhythmic activities in the visual cortex in a frequency-specific fashion and induce visual experiences (phosphenes). We delivered an oscillatory current over the occipital cortex with tACS. In order to observe interactions with ongoing cortical rhythms, we compared the effects of delivering tACS under conditions of light ("Light" condition) or darkness ("Dark" condition). Stimulation over the occipital cortex induced perception of continuously flickering light most effectively when the beta frequency range was applied in an illuminated room, whereas the most effective stimulation frequency shifted to the alpha frequency range during testing in darkness. Stimulation with theta or gamma frequencies did not produce any visual phenomena. The shift of the effective stimulation frequency indicates that the frequency dependency is caused by interactions with ongoing oscillatory activity in the stimulated cortex. Our results suggest that tACS can be used as a noninvasive tool for establishing a causal link between rhythmic cortical activities and their functions.