"Mercy" is a song performed by Welsh singer Duffy, released as the second single from her debut studio album, Rockferry (2008). Co-written by Duffy and Steve Booker and produced by Booker, it was released worldwide in 2008 to critical acclaim and unprecedented chart success. As Duffy's first international release, the song is credited with firmly establishing her career and is now considered her signature song. "Mercy" received comparisons to Duffy's previous single, "Rockferry". 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. "Mercy" peaked at number one on the UK Singles Chart in February 2008, remaining at the top of the chart for five weeks. The single also topped the charts in Austria, Germany, Greece, the Netherlands, Norway, Republic of Ireland, Switzerland and Turkey, and peaked within the top five of the charts in Belgium, Denmark, France, Italy, Japan, New Zealand, Romania, Spain and Sweden.

"Mercy" was nominated for several awards in 2008, including the Grammy Award for Best Female Pop Vocal Performance at the 51st Grammy Awards. Well received by the public, "Mercy" went on to become the third-best-selling single of 2008 in the United Kingdom, with sales of over 500,000 copies in the UK that year. The song achieved worldwide chart success, with Duffy attributing this 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". It is Duffy's best-selling single to date.

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. The Glee version charted on the UK Singles Chart and in Ireland, and Duffy herself performed the song for the Live Lounge. The song was performed live at various music festivals as well as on Duffy's 2008–09 tour. The main-release music video, directed by Daniel Wolfe, has been viewed on video sharing website YouTube over 60 million times.

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