"Look What You Made Me Do" is a song by the American singer-songwriter Taylor Swift and the lead single from her sixth studio album, Reputation (2017). Big Machine Records released the song on August 24, 2017, after approximately one year of Swift's hiatus due to the controversies that affected her "America's Sweetheart" public image throughout 2016. While secluding from public appearances, she wrote and produced the track with Jack Antonoff.

"Look What You Made Me Do" has an electronic production combining electropop, dance-pop, progressive pop, and synth-punk with elements of hip hop, electroclash, industrial, and electro. It contains an interpolation of "I'm Too Sexy" (1991) by the English pop group Right Said Fred, whose members received songwriting credits as a result. The melody incorporates strings, plinking piano, and synthesizers, and the chorus consists of drumbeats and rhythmic chants. The lyrics are about a narrator's contempt for somebody who had wronged them; many media publications interpreted the track to be a reference to the controversies that Swift faced.

The accompanying music video premiered at the 2017 MTV Video Music Awards and contains various implications of Swift's celebrity that received widespread media speculation. Both the song and the video broke streaming records on Spotify and YouTube upon release. "Look What You Made Me Do" polarized music critics: some deemed it a fierce return and an interesting direction but others criticized the sound and theme as harsh and vindictive that strayed away from Swift's singer-songwriter artistry. Critics have considered "Look What You Made Me Do" a career-defining single for Swift.

In the United States, the single peaked atop the Billboard Hot 100 with the highest sales week of 2017 and was certified four-times platinum by the Recording Industry Association of America. The single also peaked atop the singles charts of countries including Australia, Canada, Ireland, the Philippines, and the United Kingdom, and it received multi-platinum certifications in Australia, Brazil, Canada, Poland, Sweden, and the United Kingdom. Swift performed the song on the Reputation Stadium Tour (2018) and the Eras Tour (2023–2024).