"Sweetest Sin" is a song by American recording artist 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 (2003). An accompanying music video, directed by Constantine Paraskavopoulos, showed Simpson swaying on a beach and flirting with singer Nick Lachey during the whole video.

"Sweetest Sin" was written by Diane Warren and produced by Ric Wake. According to the sheetmusic published at Music Notes by Real Songs (ASCAP), the R&B song is composed in the key of D minor and is set in the time signature of common time with a tempo of 96 beats per minute. It has the sequence of C–D–B♭ as its chord progression. Simpson's vocals in the song span from the note of D₄ to the high note of F₅.

The recording process of "Sweetest Sin" was documented in an episode of Simpson's reality series, Newlyweds: Nick and Jessica. Simpson originally recorded "Sweetest Sin" confidentially in a slightly remixed form and announced she was pleased with its progress. However, her superiors at Columbia Records told her that she had made the song too "challenging"; in essence, they felt that potential listeners of the song would be alienated by singing they would not be able to duplicate. Simpson was filmed breaking into tears when she received the news. Later, she re-recorded her vocals with the assistance of her then-husband, Nick Lachey, whom she brought along because she felt he was a good vocal producer.

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