"Sweetest Sin" is a song by American singer Jessica Simpson. It was written by Diane Warren and produced by Ric Wake for Simpson's third studio album, In This Skin (2003). It was released as the album's first single on July 22, 2003 through Columbia Records. The R&B song, speaks about what would be the sweetest sin between a couple. "Sweetest Sin" received positive reviews from critics, with some calling it a catchy tune and an "ode to booty knocking". Simpson performed the song on her Reality Tour Live (2004). An accompanying music video, directed by Constantine Paraskavopoulos, showed Simpson swaying on a beach and flirting with singer Nick Lachey during the whole video.

Jessica Ann Simpson is an American singer, actress, and businesswoman. After performing in church choirs as a child, Simpson signed with Columbia Records in 1997, aged seventeen. Her debut studio album, Sweet Kisses (1999), sold two million copies in the United States and was led by the Billboard Hot 100-top three single "I Wanna Love You Forever". Simpson adopted a more mature image for her second studio album, Irresistible (2001), and its namesake lead single track peaked within the top 20 of the chart. The album received gold certification by the Recording Industry Association of America (RIAA). In This Skin (2003), Simpson's third studio album, sold three million copies in the United States.