Decemberunderground was released on May 29, in the United Kingdom, June 6, in the United States, and June 16 in Mexico. The album's lead single "Miss Murder" was scheduled to premiere on LIVE 105 on April 13, 2006, but due to the thirty-second preview that aired on April 11 receiving such an overwhelming response, Interscope Records gave the station permission to play the entire track. The full track listing of the album was announced on April 25, 2006.
Customers who pre-ordered the album at special pre-listening events received a CD including the "Miss Murder" single, which contained a cover of INXS's song "Don't Change". The first pressing of the CD in the United States contained limited edition artwork of a single portrait of each band member and a portrait of the entire band on the reverse insert, serving as the CD cover and hiding the original portrait on the CD booklet cover. The album was re-released on November 7, 2006, on 6 "icy clear" 7" vinyl discs, featuring a picture of each member and the band's manager Smith Puget, who is referred to as '...a very special guest.'
AFI began touring in support of Decemberunderground in April 2006 and held their last concert in promotion of the album at Download Fest in October 2007, as Havok and Puget began to promote their side-project Blaqk Audio before beginning work on the band's eighth studio album Crash Love.
Decemberunderground received generally positive reviews. The review aggregator Metacritic awarded the album a 72 out of 100, based on 15 reviews.
Entertainment Weekly awarded the album a 'B+,' and called it "the most melodically acute distillation of that style yet," unlike their previous studio album Sing the Sorrow, which they awarded a negative 'D.' Allmusic awarded the album 4 out of 5 stars, saying the "core of AFI's sound never strays too far from what listeners have grown to love about them in the first place."
Rolling Stone awarded the album 3 out of 5 stars, one less star than their review of Sing the Sorrow, and claimed that "something isn't right in the world of AFI," because of their pop-rock musical structure "that would be at home on the soundtrack to the next Spider-Man movie".
All tracks recorded during the Decemberunderground sessions unless otherwise noted.
"Rabbits are Roadkill on Rt. 37" was recorded during the Sing the Sorrow sessions, and was eventually released on the MySpace Records: Volume 1 compilation and as a bonus track on the UK and Australian editions of Decemberunderground.