Our environment is dominated by regular structure, particularly in the auditory domain (speech, music, dance). Previous research has shown that rhythmic neural activity, also called neural oscillations, adjusts to this input rhythm, creating stimulus-brain alignment, or neural entrainment. Although neural entrainment has been associated with improved speech comprehension, it remained unclear whether it is indeed causally relevant for speech perception; i.e. if changes in neural entrainment cause changes in speech perception. This question was addressed in the current project. We manipulated entrainment using a brain stimulation technique shown to influence neural oscillations (transcranial alternating current stimulation, or tACS), and measured consequences for both neural activity and perception in human participants. We found that stimulus-brain alignment indeed causally modulates speech processing. This research therefore reveals potential novel treatment of clinical conditions such as hearing impairment.