Hemi-Sync is a trademarked brand name for a process used to create audio patterns containing binaural beats, which are commercialized in the form of audio CDs. Other companies have also created and marketed products based on the concept of binaural beats such as Holosync by the Centerpointe Institute.

Hemi-sync is a patented technology developed by Robert Monroe. Hemi-sync is short for Hemispheric Synchronization aka Brainwave synchronization. Monroe indicated that the technique synchronizes the two hemispheres of one's brain, thereby creating a 'frequency-following response' designed to evoke certain effects. Hemi-Sync has been used for many purposes, including relaxation and sleep induction, learning and memory aids, helping those with physical and mental difficulties, and reaching altered states of consciousness through the use of sound.

The technique involves using sound waves to entrain brain waves. Wearing headphones, Monroe claimed that brains respond by producing a third sound (called binaural beats) that encouraged various brainwave activity changes.

Replicated, double-blind, randomised trials on anesthetised patients have found Hemi-Sync effective as a partial replacement for fentanyl during surgery. A similar study found it ineffective at replacing propofol however.