The Order of Nine Angles (ONA or O9A) is a militant Satanic left-hand path occultist and terrorist network that originated in the United Kingdom but has since branched out into other parts of the world. Claiming to have been established in the 1960s, it rose to public recognition in the early 1980s, attracting attention for its neo-Nazi ideology and activism. Describing its approach as "Traditional Satanism", it also exhibits Hermetic and modern Pagan elements in its beliefs.

According to the Order's own claims, it was established in the Welsh Marches of Western England during the late 1960s by a woman who had previously been involved in a secretive pre-Christian sect which survived in the region. This account also states that in 1973 a man named "Anton Long" was initiated into the group, subsequently becoming its grand master. Several academic commentators who have studied the ONA express the view that the name "Anton Long" is probably the pseudonym of the British neo-Nazi activist David Myatt, although Myatt has denied that this is the case. From the late 1970s onward, Long authored books and articles which propagated the Order's ideas, and in 1988 it began publishing its own journal, Fenrir. Through these ventures it established links with other neo-Nazi Satanist groups around the world, furthering its cause through embracing the Internet in the 2000s.

The ONA advocates accelerationism and promotes the idea that human history can be divided into a series of aeons, each of which contains a corresponding human civilization. It expresses the view that the current aeonic civilization is that of the Western world, but it claims that the evolution of this society is threatened by the "Magian/Nazarene" influence of the Judeo-Christian-Islamic religion, which the Order seeks to combat in order to establish a militaristic new social order, which it calls the "Imperium". According to Order teachings, this is necessary in order for a galactic civilization to form, in which "Aryan" society will colonise the Milky Way. It advocates a spiritual path in which practitioners are required to break societal taboos by isolating themselves from society, committing crimes, embracing political extremism and violence, and carrying out acts of human sacrifice. ONA members practice magic, believing that they are able to do it by channeling energies into their own "causal" realm from an "acausal" realm where the laws of physics do not apply, and these magical actions are designed to help them achieve their ultimate goal of establishing the Imperium.

The ONA eschews any central authority or structure; instead, it operates as a broad network of associates – termed the "kollective" – who are inspired by the texts which were originally authored by Long and other members of the "inner ONA". The group is composed largely of clandestine cells, which are called "nexions". Some academic estimates suggest that the number of individuals who are broadly associated with the Order falls in the low thousands. Various rapes, killings and acts of terrorism have been perpetrated by far-right individuals influenced by the ONA, with various British politicians and activists calling for the ONA to be proscribed as a terrorist group.