Underground music is music with practices perceived as outside, or somehow opposed to, mainstream popular music culture. Underground music is intimately tied to popular music culture as a whole, so there are important tensions within underground music because it appears to both assimilate and resist the forms and processes of popular music culture.

Underground music may be perceived as expressing sincerity, intimacy, freedom of creative expression in opposition to those practices deemed formulaic or commercially driven. Notions of individuality non-conformity are also commonly deployed in extolling the virtue of underground music. There are examples of underground music that are particularly difficult to encounter, such as the underground rock scenes in the pre-Mikhail Gorbachev Soviet Union, in which has amassed a devoted following over the years (most notably for bands such as Kino). However, most underground music is readily accessible, although performances and recordings may be difficult for the uninitiated to find.

Some underground styles eventually became mainstream, commercialized pop styles, such as the underground hip hop style of the early 1980s. In the 2000s, the increasing availability of the Internet and digital music technologies has made underground music easier to distribute using streaming audio and podcasts. Some experts in cultural studies now argue that "there is no underground" because the Internet has made what was underground music accessible to everyone at the click of a mouse. A current example of an underground internet music genre is Vaporwave. One expert, Martin Raymond, of London-based company The Future Laboratory, commented in an article in The Independent, saying trends in music, art, and politics are:

... now transmitted laterally and collaboratively via the internet. You once had a series of gatekeepers in the adoption of a trend: the innovator, the early adopter, the late adopter, the early mainstream, the late mainstream, and finally the conservative. But now it goes straight from the innovator to the mainstream.