Black Flag is an American punk rock band formed in 1976 in Hermosa Beach, California. Initially called Panic, the band was established by Greg Ginn, the guitarist, primary songwriter, and sole continuous member, and singer Keith Morris. They are widely considered to be one of the first hardcore punk bands, as well as one of the pioneers of post-hardcore. After breaking up in 1986, Black Flag reunited in 2003 and again in 2013. The second reunion lasted well over a year, during which they released their first studio album in nearly three decades, What The... (2013). The band announced their third reunion in January 2019.

Black Flag's sound mixed the raw simplicity of the Ramones with atonal guitar solos and, in later years, frequent tempo shifts. The lyrics were written mostly by Ginn, and like other punk bands of the late 1970s and early 1980s, Black Flag voiced an anti-authoritarian and nonconformist message, in songs punctuated with descriptions of social isolation, neurosis, poverty, and paranoia. These themes were explored further when Henry Rollins joined the band as lead singer in 1981. Most of the band's material was released on Ginn's independent record label SST Records.

Over the course of the 1980s, Black Flag's sound, as well as their notoriety, evolved. In addition to being central to the creation of hardcore punk, they were innovators in the first wave of American West Coast punk rock and are considered a key influence on punk subculture in the United States and abroad. Along with being among the earliest punk rock groups to incorporate elements and the influence of heavy metal melodies and rhythm, there were often overt free jazz and contemporary classical elements in their sound, especially in Ginn's guitar playing, and the band interspersed records and performances with instrumentals throughout their career. They also played longer, slower, and more complex songs at a time when other bands in their milieu performed a raw, fast, three-chord format.

Black Flag has been well-respected within the punk subculture, primarily for their tireless promotion of an autonomous DIY punk ethic and aesthetic. They are often regarded as pioneers in the movement of underground do-it-yourself record labels. By way of constant touring throughout the United States and Canada, and occasionally Europe, Black Flag established a dedicated cult following.