"Mercy" is a song performed by the British soul singer Duffy, released as the second single from her debut studio album, Rockferry (2008).

Critical reviewers of "Mercy" noted similarities between the song to releases by Aretha Franklin, Dusty Springfield and the Supremes, as well as contemporaries such as fellow British singer Amy Winehouse.

Duffy attributed the chart success to the fact that "everyone is searching for liberty ... from themselves or from the world they’ve created around them" and "everyone would like to be set free".

The song has been covered several times since its original release, most notably by the cast of American musical television series Glee as well as by both The Fratellis and OneRepublic on BBC Radio 1's Live Lounge.

Duffy had already written the lyrics to "Mercy" before her meeting with Steve Booker, although she has since said that she "never [has] a plan as that always fails", in reference to the writing of the song.

[5] Duffy has noted that during recording with Booker, "Mercy" "was like this melodic poem in my mind, which I just had to get out, and I knew exactly what I wanted it to sound like".

[9] In an interview with HitQuarters Booker said he searching for a song that you could imagine sending the audience crazy on the 1960s UK pop show Ready Steady Go!.

Mark Edward Nero of About.com noted that "[although] the song sets the stage for who Duffy is as an artist as well as a person" it was "still fairly tame, lyrically".

"Mercy" garnered overwhelming acclaim from critics, often being praised for its catchy bass line and "summery", jazzy themes.

[13] The Times in a review for Rockferry said that "Mercy" is "a slight but darned catchy number" and "a classic radio hit".

[14] The Independent said that "Mercy" and "Rockferry" "are tracks that stand out as something special", whilst noting that the "uptempo organ funk" provides "the album's one truly memorable groove".

[16] The Observer, in an extremely positive review for Rockferry, said that "Mercy" "is a big, booming, finger-wagging sashay worthy of the Supremes".

[18] NME, giving Rockferry a negative review, had a mixed attitude towards "Mercy", comparing the song's "Austin Powers organ" to work by Aretha Franklin.

[5] MOJO editor Phil Alexander called the song a "timeless" classic that "could really have come from any period in time over the last 50 years".

[25] Duffy's performance of "Warwick Avenue" at the ceremony helped to increase sales of "Mercy" by double the market average before the broadcast.

[29] "Mercy" was also nominated for PRS for Most Performed Work and Best-selling British Song at the 2008 Ivor Novello Awards,[30] winning in the former category.

[31] The song was the fourth most played anywhere in the United Kingdom in 2009, revealed in a list compiled by music body PRS.

It remained at the top of the charts for five weeks until it was dethroned by "American Boy" by Estelle featuring Kanye West.

[43] In the United States overall, "Mercy" was certified Platinum for sales of one million copies by the American recording industry association, the RIAA.

[59][60] The main-release music video for "Mercy", directed by Daniel Wolfe,[61] features Duffy standing on top of a platform performing the song.

The US version was given a positive review by Nick Levine of Digital Spy, who wrote that "It's glossy, full of young, good-looking types and proudly features a drum with Duffy's name printed on its skin – hey, that's success for you.

[68] OneRepublic then recorded a studio version of "Mercy" as a bonus track for their second album Waking Up (note that the track is not to be confused with another OneRepublic song and single, also titled "Mercy" and released on their debut studio album, Dreaming Out Loud).

The Fratellis also recorded "Mercy", mixed with the Minder theme tune, specially for broadcast on the Live Lounge.