Tertia is the second full-length studio album by the American post-rock band Caspian, released through The Mylene Sheath digitally on August 11, 2009, and physically on September 15, 2009.

[5][6] It is the band's first album to be released on The Mylene Sheath, after previously issuing the vinyl versions of The Four Trees and You Are the Conductor via the company.

[5] Tertia was made available as a full free download on the same day as its digital release, through Gimme Sound.

[9] The album was produced by Ethan Dussault with assistance from Ed Llerena at New Alliance in Cambridge, Massachusetts.

Nate Shumaker (who worked on You Are the Conductor) and PAJ returned on design duties for the project.