THX is a suite of high fidelity audiovisual reproduction standards for movie theaters, screening rooms, home theaters, computer speakers, video game consoles, car audio systems, and video games. The famous THX trailer that precedes movies is based on the Deep Note, with a distinctive glissando up from a rumbling low pitch.

THX was developed by Tomlinson Holman at George Lucas's company Lucasfilm in 1983 to ensure that the soundtrack for the third Star Wars film, Return of the Jedi, would be accurately reproduced in the best venues. THX was named after Holman's initials, with the "X" standing for "crossover" or "experiment". The name is also an homage to Lucas's first film, THX 1138 (1971). Deep Note was created by Holman's co-worker James A. Moorer. THX Ltd. was founded on May 20, 1983 by Lucas and Holman, and headquartered in San Francisco, California.

THX is a quality assurance system, not a recording technology. All sound recording formats, whether digital (Dolby Digital, DTS, SDDS) or analog (Dolby Stereo, Ultra Stereo), can be reproduced in a THX system. THX-certified theaters provide a high-quality, predictable playback environment to ensure that any film soundtrack mixed in THX will sound as near as possible to the intentions of the mixing engineer. THX also provides certified theaters with a special crossover circuit whose use is part of the standard. Certification of an auditorium entails specific acoustic and other technical requirements; architectural requirements include a floating floor, baffled and acoustically treated walls, non-parallel walls (to reduce standing waves), a perforated screen (to allow center channel continuity), and NC30 rating for background noise ("ensures noise from air conditioning units and projection equipment does not mask the subtle effects in a movie's soundtrack").

On June 12, 2002, THX was spun off as a separate company from Lucasfilm and sold to sound card manufacturer Creative Technology Limited, which held a 60% share of the company. Under Creative Technology, the company developed several further innovations, such as the first THX-certified audio card for computers, the Sound Blaster Audigy 2. In 2016, THX was acquired by video game hardware company Razer Inc.