Lip sync or lip synch (pronounced , the same as the word sink, short for lip synchronization) is a technical term for matching a speaking or singing person's lip movements with sung or spoken vocals.

Audio for lip syncing is generated through the sound reinforcement system in a live performance or via television, computer, cinema speakers, or other forms of audio output. The term can refer to any of a number of different techniques and processes, in the context of live performances and audiovisual recordings.

In film production, lip syncing is often part of the post-production phase. Dubbing foreign-language films and making animated characters appear to speak both require elaborate lip syncing. Many video games make extensive use of lip-synced sound files to create an immersive environment in which on-screen characters appear to be speaking. In the music industry, lip syncing is used by singers for music videos, television and film appearances and some types of live performances. Lip syncing by singers can be controversial to fans attending concert performances who expect to view a live performance.