Altar is a collaboration album between experimental music groups Boris and Sunn O))), originally released on October 31, 2006 through Southern Lord Records (SUNN62). in addition to major players Sunn O))) and Boris.

Altar also boasts an extensive roster of guest musicians/collaborators such as Kim Thayil (Soundgarden), Joe Preston (Earth, Thrones, Melvins, High on Fire), Phil Wandscher and Jesse Sykes (both of Jesse Sykes and the Sweet Hereafter) as well as long time Sunn O))) collaborators TOS Niewenhuizen and Rex Ritter.