A declaration of war is a formal act by which one state announces to war against another.

Declaration of war may also refer to:

Declaration of War (album), a 1993 album by RaHoWa Declaration of War (film), a 2011 French film Declaration of War (horse), an American racehorse "Declaration of War" (song), a 2008 song by Hadouken!

See also State of War (disambiguation)