Don't Break the Oath is the second studio album by Danish heavy metal band Mercyful Fate, released on 7 September 1984 through Roadrunner Records.

The style Mercyful Fate employed on Don't Break the Oath resembled a mixture of heavy metal with progressive elements, lyrically preoccupied with Satan and the occult and distinguished by King Diamond's theatrical falsetto vocals. Very influential to future black metal bands due to its lyrical content, the music itself tends toward progressive forms such as dramatic modulations, tempo and tone changes. The album was remastered and subsequently re-issued on Roadrunner Records in 1997. This reissue came with the bonus track "Death Kiss (Demo)", which would eventually evolve into the album's lead-off track, "A Dangerous Meeting". The album received critical acclaim, and Metal Rules named this the greatest extreme metal album of all time.