Thrash metal (or simply thrash) is an extreme subgenre of heavy metal music characterized by its overall aggression and fast tempo. The songs usually use fast percussive beats and low-register guitar riffs, overlaid with shredding-style lead guitar work.

The genre emerged in the early 1980s as musicians began fusing the double bass drumming and complex guitar stylings of the new wave of British heavy metal (NWOBHM) with the speed and aggression of hardcore punk and speed metal and the technicality of progressive rock. Philosophically, thrash metal developed as a backlash against both the conservatism of the Reagan era and the much more moderate, pop-influenced, and widely accessible heavy metal subgenre of glam metal which also developed concurrently in the 1980s. Derived genres include crossover thrash, a fusion of thrash metal and hardcore punk.

The early thrash metal movement revolved around independent record labels, including Megaforce, Metal Blade, Combat, Roadrunner, and Noise, and the underground tape trading industry in both Europe and North America. The genre was commercially successful from approximately 1985 through 1991, bringing prominence to Metallica, Slayer, Megadeth, and Anthrax, all grouped together as the "Big Four" of U.S. thrash metal. Other bands, such as Overkill, Metal Church, Nuclear Assault, Flotsam and Jetsam, and Bay Area acts Exodus, Testament and Death Angel, never achieved the same level of success as the "Big Four" but had also developed a strong following in the metal community, through MTV's Headbangers Ball or otherwise. Some of the most popular international thrash metal bands from this era were Brazil's Sepultura, Canada's Voivod and Annihilator, Switzerland's Coroner, England's Onslaught, and the genre's German "Big Four": Kreator, Destruction, Sodom, and Tankard.

The thrash metal genre had declined in popularity by the mid-1990s, due to the commercial success of numerous genres such as alternative rock, grunge, and later pop-punk and nu metal. In response, some bands either disbanded or moved away from their thrash metal roots and more towards groove metal or alternative metal. The genre has seen a resurgence in popularity since the 2000s, with the arrival of various bands such as Bonded by Blood, Evile, Hatchet, Havok, Lamb of God, Municipal Waste, and Warbringer, who have all been credited for leading the so-called "thrash metal revival" scene.

Thrash metal generally features fast tempos, low-register, complex guitar riffs, high-register guitar solos, and double bass drumming. The rhythm guitar parts are played with heavy distortion, power chords and often palm muted to create a tighter and more precise sound. Vocally, thrash metal can employ anything from melodic singing to shouted or screamed vocals. Most guitar solos are played at high speed and technically demanding, as they are usually characterized by shredding, and use advanced techniques such as sweep picking, legato phrasing, alternate picking, tremolo picking, string skipping, and two-hand tapping.

David Ellefson, the original bassist of Megadeth, described thrash metal as "a combination of the attitude from punk rock but the riffs and complexities of traditional metal." On the origins of thrash metal, Dan Lilker (bassist and co-founding member of Anthrax, S.O.D. and Nuclear Assault) recalls: "Thrash was just what they called 'faster hardcore,' because you literally thrashed around when you were either playing it or reacting to it. And thrash metal was born because it was influenced by thrash hardcore, and they just thought it was more metal, so they said, 'Okay, this is thrash metal.'"

The guitar riffs often use chromatic scales and emphasize the tritone and diminished intervals, instead of using conventional single-scale-based riffing. For example, the intro riff of Metallica's "Master of Puppets" (the title track of the namesake album) is a chromatic descent, followed by a chromatic ascent based on the tritone.

Speed, pacing, and time changes also define thrash metal. Thrash tends to have an accelerating feel which may be due in large part to its aggressive drumming style. For example, drummers often use two bass drums, or a double-bass pedal to create a relentless, driving beat. Cymbal stops/chokes are often used to transition from one riff to another or to precede an acceleration in tempo. Some common characteristics of the genre are fast guitar riffs with aggressive picking styles and fast guitar solos, and extensive use of two bass drums as opposed to the conventional use of only one, typical of most rock music.