Memory consolidation during sleep is thought to depend on coordinated interplay of cortical slow waves, sleep spindles, and hippocampal ripples, but direct evidence is lacking. I developed a real-time, closed-loop system triggering intra-cortical electrical stimulation based on brain activity, that will be used to study the neurophysiology of memory consolidation processes. Synchronizing stimulation, locked to slow waves in medial temporal lobe (MTL), but not identical stimulation without precise time-locking, enhanced slow waves and sleep spindles, increased locking of brain-wide neural activity to MTL slow waves, and improved coupling between hippocampal-ripples and neocortical oscillations. Additionally, synchronizing stimulation improved memory performance in a manner highly correlated with electrophysiological effects. Our results establish that hippocampo-cortical synchronization during sleep causally supports human memory consolidation via coordination between ripples and neocortical oscillations, and suggest potential avenues for treatment of memory disorders during sleep.