"Look What You Made Me Do" is a song recorded by American singer-songwriter Taylor Swift, released on August 24, 2017, by Big Machine Records, as the lead single from her sixth studio album Reputation. Swift wrote and produced the song with her co-producer Jack Antonoff. "Look What You Made Me Do" is a dance-pop, electropop, and electroclash song, with lyrics about various issues that built Swift's reputation. Right Said Fred members Richard Fairbrass, Fred Fairbrass, and Rob Manzoli received songwriting credits because its hook interpolates the melody of their 1991 song "I'm Too Sexy". Released by Swift after a year of public hiatus, several publications have noted the song to be one of pop music's most memorable comebacks, buoyed by its accompanying music video.

Breaking a string of records, "Look What You Made Me Do" became Swift's fifth number-one single on the US Billboard Hot 100. The song amassed the most plays in a single day on Spotify and topped the Hot 100 for three consecutive weeks. It also debuted atop the Billboard Digital Song Sales and Streaming Songs charts, with 353,000 song downloads and 84.4 million streams, respectively. The song debuted and peaked at number-one in several countries worldwide, including Australia, Canada, Ireland, Japan, Malaysia, New Zealand, the Philippines, Scotland, and the United Kingdom. The song received polarized reviews from music critics—some complimented Swift's new direction and praised it as a fierce return, while others labeled the change of style a disappointment.

The music video, directed by Joseph Kahn, premiered at the 2017 MTV Video Music Awards, three days after the song's release. Upon its release on YouTube, the music video garnered 43.2 million views in its first 24 hours, breaking the record for the most views in its first 24 hours of release, at that time; it also broke the 24-hour Vevo record, which is now held by Swift's own 2019 single "Me!". The video garnered critical acclaim and was ranked by Billboard and Rolling Stone as one of the best music videos of 2017. As of July 2018, "Look What You Made Me Do" is certified 4× Platinum by the Recording Industry Association of America for exceeding 4 million units in the US. The song has also received platinum or multi-platinum certifications in Australia, Belgium, Canada, Denmark, Italy, New Zealand, Poland, and Sweden, and a diamond certification in Brazil.