Old Man Gloom (In Full: The Old Man Gloom Alien Simian Defence League) is a sludge/metal band, with many ambient and soundscape elements. They originally formed in New Mexico, United States, but are now based in Massachusetts. The group, formed by Aaron Turner of Isis and Santos Montano have now expanded to become a sort of supergroup in the Boston hardcore and metalcore scene. The group is less a band than it is a recording project for a variety of local talent. By the release of their first album (Meditations in B), Old Man Gloom expanded to include two new members, creating the following lineup:

Aaron Turner (also of Isis): lead vocals, guitar
Santos Montano: drums
Nate Newton (also of Converge): guitar, vocals
Caleb Scofield (also of Cave In): bass guitar, vocals

In 2001, a year after the release of Meditations in B, the band released two albums simultaneously: Seminar II and Seminar III. For these records, Luke Scarola joined to use electronics. On Seminar II, Steven Brodsky of Cave In wrote the lyrics for one song, and Agnostic Blunt (aka Jay Randall of Agoraphobic Nosebleed) contributed on electronics. On August 24, 2004, their most recent album, Christmas, was released.