Metallica is considered one of the pioneering bands of the thrash metal genre and one of the founding "big four" bands of thrash metal, alongside Megadeth, Anthrax, and Slayer. The bands sound has been described as heavy metal, thrash metal, speed metal, and hard rock. Metallicas early releases contained fast tempos, harmonized leads, and extended, progressive epics. The band was influenced by early heavy metal and hard rock bands and artists, new wave of British heavy metal (NWOBHM) bands, and European bands. Early punk rock bands and hardcore punk acts also influenced Metallicas style. Metallica experimented with different musical paths and incorporated styles from hard rock and even alternative rock into some of their works. However, they are primarily recognized as a metal band.

In summary, Metallica is considered a heavy metal band and one of the pioneering bands of the thrash metal genre, but their sound has also been described as hard rock, speed metal, and thrash metal.