"Fortnight" is a song by the American singer-songwriter Taylor Swift featuring the American rapper and singer Post Malone. It was written by the artists alongside Jack Antonoff, who produced it with Swift. "Fortnight" is a 1980s-inspired downtempo, electropop, synth-pop, and new wave ballad built around a steady, pulsing synth bassline, concluding with a bridge-like outro blending Swift and Malone's vocal harmonies. The lyrics describe an emotionally impactful romance that lasts for two weeks: a woman in an unhappy marriage rekindles with a married ex-lover, and the two vow to escape to Florida.

Republic Records released "Fortnight" to mainstream radio as the lead single from Swift's eleventh studio album, The Tortured Poets Department, on April 19, 2024. Reviews mostly praised the vocal chemistry between Swift and Malone, but some found the composition dull. The song broke the record for the highest single-day streams on Spotify and peaked atop the Billboard Hot 100 in the United States, the Billboard Global 200 chart, and the charts in Australia, Canada, Malaysia, New Zealand, the Philippines, Singapore, the United Arab Emirates, and the United Kingdom. At the 67th Annual Grammy Awards, "Fortnight" was nominated for Song of the Year and Record of the Year.

Swift directed the music video for "Fortnight", collaborating with the Mexican cinematographer Rodrigo Prieto. The video is predominantly black-and-white video, with Swift playing a mentally deranged patient portrayed in Victorian-gothic fashion who mourns a past relationship and relives its memories. It stars Malone as her lover, and the Dead Poets Society co-stars Ethan Hawke and Josh Charles as mad scientists experimenting on her. The video was met with critical praise and garnered a number of accolades, such as the Best Music Video at the 32nd Camerimage festival, five wins at the 2024 MTV Video Music Awards (including Video of the Year), the Best Video at the 2024 MTV Europe Music Awards, and a Best Music Video nomination at the 67th Annual Grammy Awards.

Taylor Swift developed her eleventh original studio album, The Tortured Poets Department, immediately after finishing her previous album, Midnights (2022). She continued working on The Tortured Poets Department while embarking on the Eras Tour in 2023, amidst media reports on her personal life, including a breakup after a long-term relationship with Joe Alwyn and a short-lived romantic relationship with Matty Healy.

Swift announced The Tortured Poets Department at the 66th Annual Grammy Awards on February 4, 2024, and unveiled the track listing via social media the following day. "Fortnight" is the opening track on the album, with Post Malone as the featured artist. Swift brought the track to Malone's home studio in Los Angeles, and he recorded his background vocals and additional hooks for the bridge. During an interview with Zane Lowe for Apple Music 1 in February 2024, Malone said that he had not heard the full song.

"Fortnight" was released as the lead single of The Tortured Poets Department on April 19, 2024, concurrently with the album's release; it was released to United States contemporary hit radio and hot adult contemporary radio by Republic Records, Italian radio airplay by Universal Music Group, and Canadian radio airplay by Republic/Universal. Swift unveiled the cover artwork via social media; it is a grayscale photograph of Swift resting her cheek on a closed fist and Malone gazing into the camera with his hands clasped. The track was made available for purchase as a limited-time CD single through Swift's online store in the United States, Ireland, the United Kingdom, Germany, and Switzerland. A remix by Blond:ish was released on May 21, and an acoustic version and a remix by Cults followed on July 9, 2024. A special 7-inch vinyl edition was released on April 12, 2025, to commemorate Record Store Day.

Starting from the Eras Tour shows in May 2024 in Paris, Swift revamped the set list to include songs from The Tortured Poets Department, including "Fortnight". For the performance of "Fortnight", the stage featured a "TTPD"-emblazoned bed and dancers dressed as nurses. Swift sang the song as she sat on a typewriter, across from a dancer, and the bed, which also resembled an office desk, moved across the stage.

Swift and Malone wrote "Fortnight" with Jack Antonoff, who programmed the track and played acoustic guitar, electric guitar, the Juno, the M1, drums, and percussion. Sean Hutchinson played additional drums, which were recorded by himself and Michael Riddleberger at Hutchinson Sounds in Brooklyn. Malone's vocals were recorded and produced by Louis Bell at Electric Feel Studios in Los Angeles. "Fortnight" was produced by Swift and Antonoff; recorded by Laura Sisk and Oli Jacobs at Conway Recording Studios in Los Angeles and Electric Lady Studios in New York City; and mixed by Serban Ghenea at Mixstar Studios in Virginia Beach.