Isis (sometimes stylized ISIS) was an American post-metal band formed in Boston, Massachusetts in 1997 by guitarist and vocalist Aaron Turner, bassist Jeff Caxide, vocalist and electronic instrumentalist Chris Mereschuk and drummer Aaron Harris. After a demo and the EP Mosquito Control were recorded by the original lineup, Mereschuk was replaced by Jay Randall in 1999, who joined the group alongside guitarist Michael Gallagher. Jay Randall would later be replaced by guitarist and keyboardist Bryant Clifford Meyer after the recording of Red Sea. With roots in hardcore punk and doom metal, the band borrowed from and helped to evolve the post-metal sound pioneered by bands such as Neurosis and Godflesh, characterized by lengthy songs focusing on repetition and evolution of structure. Isis disbanded in June 2010, just before the release of a split EP with the Melvins, reforming only once in 2018 as Celestial for a one-off show to pay tribute to Caleb Scofield.

Initially releasing their debut EP Mosquito Control (1998) under Escape Artist Records, their debut studio album Celestial (2000) under Turner's own label, Hydra Head Records, and its sister EP SGNL›05 (2001) under Neurot Recordings, the band would sign with Ipecac Recordings, who issued the band's subsequent material until their dissolution.

In Boston, several sessions of experimentation led friends Aaron Turner (guitar/vocals; also the owner of Hydra Head Records and its subsidiary, HH Noise Industries), Jeff Caxide (bass guitar), Chris Mereschuk (electronics/vocals) and Aaron Harris (drums) to form Isis in the autumn of 1997. As Turner stated, "Isis formed as a result of the dissatisfaction with past bands of the founding members. None of us were happy with what we were doing musically at the time, two of us lived together, we had similar tastes and similar record collections." The band began playing out in the spring of 1998, and recorded a demo at Salad Days Studios shortly thereafter. After an East Coast tour in the summer of 1998 where they were joined by Randy Larsen of Cable on guitar, Mereschuk left the band. In 1999, Michael Gallagher (formerly of Cast Iron Hike) and Jay Randall (now of Agoraphobic Nosebleed) joined the band, working on Red Sea (1999). Jay Randall's time with the band was brief, and Isis recruited guitarist/keyboardist Bryant Clifford Meyer (formerly of The Gersch) to replace him. After releasing their full-length debut entitled Celestial (2000) and its sister EP, SGNL›05 (2001, on Neurot Recordings), Isis gained national underground attention in the metal/hardcore scene through tours with Cave In and Neurosis. Isis remained with this lineup until their dissolution in 2010.

For the SGNL›05 EP, they contacted Godflesh member Justin Broadrick through their friends in Neurosis to remix the title track from Celestial, which they used as the EP's closer. Following SGNL›05, the band felt a need to expand its ambit, both artistically and in terms of distributive reach. The entire band were avid fans of Melvins, so their label – Mike Patton's Ipecac Recordings – was instantly put forward as an ideal candidate. Turner's friend James Plotkin was already working with Ipecac, so he showed some material to Patton, who, unknown to the band, was already a fan. After discussion, they signed with Ipecac, who went on to issue the band's subsequent studio albums.

Whereas Celestial was still deeply rooted in heavy metal and hardcore, 2002's follow-up, Oceanic, saw the band acquire new characteristics comparable to post-rock and ambient music, significantly aiding in the birth of the genre of post-metal in what many saw as a logical progression. While much of the material on the album retained the band's former "metallic" intensity, this departure saw the band appeal to a far wider audience; as a result, Oceanic may be the group's most noted album to date, and is widely considered a turning-point in the history of the band. Turner himself describes it as their "quintessential album". It was at the time their most successful release, receiving album-of-the-year accolades from Rock Sound and Terrorizer in 2002, In late 2003, Isis relocated to Los Angeles.

The distinctive tone of material since and including Oceanic had a noticeable impact on avant-garde metal, helping develop the sound of several contemporaries; Cult of Luna, Pelican, Tides, Rosetta, and Russian Circles all cite Isis as an influence. This underground success attracted the attention of the likes of Mogwai, with whom they have toured on numerous occasions.

Oceanic Remixes and Reinterpretations was released in 2004, featuring reinterpretations of songs from Oceanic by a number of influential artists requested by the band. Both Oceanic and Oceanic Remixes feature vocals by Maria Christopher of the band 27. The album featured another remix by Justin Broadrick, who has supported Isis on tours with his band, Jesu, which is signed to Hydra Head Records.

2004 saw the release of Isis' third album, Panopticon. It signified a further progression many had predicted since Oceanic, with a more advanced post-rock feel to the music both structurally and in terms of sound. Justin Chancellor of Tool makes an appearance on the track, "Altered Course". Overall, it was a very well received album, being awarded 'album of the year' accolades from Rock Sound and reaching No. 47 on Billboard's Top Independent Albums charts; their first entry into any mainstream charts. Before touring the United States, the band performed a free concert at the Los Angeles Museum of Contemporary Art, in a manifestation of the widespread recognition the band had acquired in artistic circles since the release of Oceanic. Reacting to the impact of Oceanic and Panopticon, Revolver named Isis the twelfth-heaviest band of all time in December 2004. On July 23, 2006, they performed Oceanic in full at KOKO in London as part of All Tomorrow's Parties Don't Look Back season.