Bay Area thrash metal (also known as Bay Area thrash) referred to a steady following of heavy metal bands in the 1980s who formed and gained international status in the San Francisco Bay Area in California. Along with Central Florida, the scene was widely regarded as a starting point of American thrash metal, crossover thrash and death metal.

The earliest documented roots of the Bay Area thrash scene date back to the formation of Exodus in 1979. By the time the group recorded their full-length album four years later, five different guitarists or bassists had already passed through the lineup, with some going on to join or form bands that were equally relevant to the area's burgeoning metal scene. In November 1982, Exodus opened a show at San Francisco's Old Waldorf venue for Metallica, a then-relatively unknown (and unsigned) band from Southern California who were recently discovered by Brian Slagel and had appeared on the first volume of his Metal Massacre compilation. Exodus (who were also unsigned at this time) had distributed an untitled demo the same year with a lineup that included guitarist Kirk Hammett.

Although Metallica had initially formed in Los Angeles, it was not until their February 1983 relocation to the East Bay area that Cliff Burton formerly from Trauma joined as bassist, sealing the band's first, formative lineup. The group (which then consisted of Burton, James Hetfield, Lars Ulrich, and Dave Mustaine) had moved into a Carlson Blvd. house in El Cerrito, near Albany Hill, which had been rented by Mark Whitaker, then-manager of Exodus. After Mustaine was removed from the lineup that same year, then-Exodus guitarist Hammett would replace him following Whitaker's recommendation; in turn, Mustaine would move back to Los Angeles to form Megadeth.

Metallica's signing to Megaforce Records, the release of their first album and the songwriting process and rehearsals for Ride the Lightning and Master of Puppets had all taken place while the group resided at Whitaker's El Cerrito home.

Burton and Hammett's friendship with other local acts, notably Oakland's Exodus and Testament, and later, San Francisco's Death Angel—among others—strongly vitalized the scene, leading to intensive touring and tape trading that would cross borders and seas, and eventually graduate to record signings.

El Sobrante's Possessed would bring a turning point to the genre with 1985's Seven Churches, regarded as the first album to cross over from thrash metal to death metal for the largely "growling vocals" and subject matter dealing with horror and the occult. In addition to the inspiration of black metal, it would predate other albums, such as Los Angeles-based Slayer's Reign in Blood and Death's San Francisco Bay Area-written and rehearsed Scream Bloody Gore, which had also been regarded as influential to thrash and death metal.

Members from Possessed had also maintained a strong network with other integral musicians of the Bay Area thrash scene: vocalist Jeff Becerra and then-guitarist Larry LaLonde had played in a short-lived speed metal band called Blizzard, which featured former Exodus bassist Carlton Melson and Desecration guitarist Danny Boland (then-Possessed drummer Mike Sus and former Possessed bassist Bob Yost had also played in Desecration, a Bay Area death/thrash band active between 1985 and 1989). Former Exodus bassist Geoff Andrews co-founded Possessed before Becerra replaced him in 1983, taking over both vocal and bass duties. Andrews went on to form the thrash band Sabertooth with Johnee McKelvey.

After Possessed's first disbanding, LaLonde went on to join Blind Illusion, which featured former Exodus members Evan McCaskey and Tim Agnello, then-Heathen members Marc Biedermann and Dave Godfrey, and Blizzard drummer Mike Miner. Blind Illusion co-founder Les Claypool (who was a high school friend of Kirk Hammett at Richmond's DeAnza High School) would unsuccessfully audition as Metallica's bassist in 1986 after Burton's bus accident death in Sweden.