"She Used to Be Mine" is a song written and recorded by American singer-songwriter Sara Bareilles for her fifth studio album, What's Inside: Songs from Waitress (2015), featuring songs from her stage adaptation of the 2007 film, Waitress. It was released to digital retailers as the lead single on September 25, 2015.

"She Used to Be Mine" is a sentimental ballad written and composed by Sara Bareilles with a duration of four minutes and ten seconds (4:10). The song is instrumented primarily by the piano and also features guitar. Bareilles' vocal performance has been described as "powerful," with a vocal range of F₃ to D₅. According to the digital sheet music published by Sony/ATV Music Publishing, the song is composed in the key of F major with a "moderately slow" tempo. Lyrically, the song discusses losing one's sense of self and struggling with changes in circumstances, sung from the perspective of the musical's protagonist, Jenna.

Madison Vain of Entertainment Weekly called the song "delicate, nostalgic, and wonderfully theatrical," in the magazine's "7 Tracks We Loved This Week" feature and praised the song as "a beautiful introduction" to the musical's story.

"She Used to Be Mine" was the most-added song on hot adult contemporary (Hot AC) radio the week of its release, according to radio monitoring organization Mediabase. The song debuted at number 40 on the Billboard Adult Pop Songs chart on the chart dated October 17, 2015.

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