"Should've Said No" is a song written and recorded by American singer-songwriter Taylor Swift. The song serves as the fifth and final single from her self-titled debut studio album. The song is about Swift addressing her former lover, who cheated on her. It became her second number-one single on the Billboard Hot Country Songs chart, and was a Top 40 hit on the Billboard Hot 100. In addition, "Should've Said No" has been certified platinum by the Recording Industry Association of America (RIAA).

The song is included in the concert film Jonas Brothers: The 3D Concert Experience, and in the soundtrack to the movie. An alternate version of the song was released on her extended play Beautiful Eyes. As of November 2014, it has sold 1.4 million copies in the United States.

"Should've Said No" is an up-tempo, in which Swift addresses a former lover who has cheated on her, telling him that he "should've said no" to the woman who has taken her place in his life ("You should've said no/You should've gone home/You should've thought twice before you let it all go/You should've known that word 'bout what you did with her would get back to me"). Swift wrote the song when she was sixteen years old. She wrote the song because "something really, really dramatic and crazy" was happening to her, and she needed "to address it in the form of music". According to the Country Weekly magazine, she was inspired to write it after she discovered that her boyfriend at the time had cheated on her. The first line that came to her was the title, and she wrote the chorus in five minutes. Swift has also said that many of the lyrics were based on actual words that she used when confronting her ex-boyfriend. She compared the song to her previous single "Picture to Burn". While "Picture to Burn" has an "angry and I'm-done-with-him attitude", "Should've Said No" is "more of a moral statement. It's an 'I love you, we were awesome and great together, but you messed this up and I would still be with you' kinda thing. You said yes, and you should've said no." The song was a last-minute addition to Taylor Swift—Swift wrote the song two days before the mastering was scheduled, and booklets were about to be printed. She then called her producer, and completed the song overnight.

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