"Last Friday Night (T.G.I.F.)" is a song by American singer Katy Perry from her third studio album, Teenage Dream. The song was written and produced by Dr. Luke and Max Martin, who also co-wrote the song with Perry and Bonnie McKee. Perry stated that she was inspired to write the track after a night of wild partying and streaking. It was released as the album's fifth single on June 6, 2011, by Capitol Records, with a remix featuring American rapper Missy Elliott released to US radio stations and digital retailers on August 8, 2011; this version was included in Teenage Dream: The Complete Confection edition. It is a dance-pop song with lyrics about drunken fun and debauchery. Some of the risque lyrics in the song are often censored in radio versions of the song. The "T.G.I.F." in the title is an acronym for "".

The song had chart success worldwide, reaching number one in Slovakia and the Czech Republic, and has attained top-ten positions in Austria, Ireland, Italy, and Poland. By topping the US Billboard Hot 100, Perry became the first female solo artist to have five number-one singles on the Hot 100 from a single album; it also was Perry's sixth number-one single on the Billboard Hot 100 chart, and her fifth consecutive number one single. The single had sold 3.7 million digital copies in the US and was the tenth most played single on US radio during 2011.

The song's accompanying music video was directed by Marc Klasfeld. The clip features Perry, as her teenage alter-ego Kathy Beth Terry, at a house party filled with music and dancing. While at the party, Terry is made over by her friend and draws attention to herself as the life of the party. The video features appearances by notable music artists and actors, which includes Rebecca Black, Corey Feldman, Debbie Gibson, Kenny G, Hanson, and Kevin McHale and Darren Criss of Glee. According to the director, the inspiration behind the video was Sixteen Candles, works by American filmmaker John Hughes, and other 1980s high school films. The music video won Favorite Music Video award at the 38th People's Choice Awards.

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