"The Lost Chord" is the title of an 1877 song composed by Arthur Sullivan.

The phrase arises from musical sounds, in particular purely harmonic or nearly harmonic chords that were "lost" to music with the change to twelve-tone equal tempered tuning, not yet completed at the time that Sullivan wrote the song. Modern microtonal musicians may use the phrase "lost chord" most often to refers to the harmonic seventh (a 7/ 4  or 7:4 chord) now replaced with a dissonant equal tempered minor seventh; a much closer pitch to 7/ 4  was formerly available via an augmented sixth in meantone temperament; there are many other "lost chords", such as those from the eleventh ( 11 /8) and thirteenth harmonics ( 13 /8).

The Lost Chord may also refer to:

• The Lost Chord (1917 film), a British film by Wilfred Noy
• The Lost Chord (1911 film), an Australian film by W. J. Lincoln
• The Lost Chord (1925 film), a lost American film by Wilfred Noy
• The Lost Chord (1933 film), a British film by Maurice Elvey
• "The Lost Chord" (The Worst Witch), a 2001 episode of The Worst Witch
• "The Lost Chord", a 2020 episode of the web series Song Machine by Gorillaz featuring Leee John
• In Search of the Lost Chord, an album by The Moody Blues, released in 1968