"I Knew You Were Trouble" is a song by the American singer-songwriter Taylor Swift from her fourth studio album, Red (2012). She wrote the song with its producers, Max Martin and Shellback. Instrumented by electric guitars and synthesizers, "I Knew You Were Trouble" has a production that incorporates pop subgenres. Its refrain is accentuated by a dubstep wobble and Swift's distorted vocals; music critics found the dubstep production a radical departure from her previous country pop sounds. The lyrics are about a narrator's self-blame after a toxic relationship upon recognizing the warning signs in hindsight.

Big Machine, in partnership with Republic Records, released "I Knew You Were Trouble" to US pop radio on November 27, 2012, as the second pop single and the third overall from Red. The song peaked within the top 10 on record charts and received multi-platinum certifications in Australia, Austria, Canada, New Zealand, and the UK. In the US, the single peaked at number two on the Billboard Hot 100 and spent seven weeks at number one on the Pop Songs chart. Its success on pop radio inspired Swift to recalibrate her artistic identity from country for pop on her 2014 album 1989. Initial reviews were divided: positive comments found the production bold, but criticisms deemed it derivative. Retrospective opinions have regarded "I Knew You Were Trouble" as one of Swift's career-defining singles.

The music video for "I Knew You Were Trouble" premiered on MTV on December 13, 2012. Directed by Anthony Mandler, it depicts Swift with an unfaithful man and ending up alone in a desert. The video won an MTV Video Music Award for Best Female Video at the 2013 MTV Video Music Awards and the Phenomenon Award at the 2013 YouTube Music Awards. Swift performed the song at awards shows including the American Music Awards, the ARIA Music Awards, and the Brit Awards. She included "I Knew You Were Trouble" on the set lists of three of her world tours: the Red Tour (2013–2014), the 1989 World Tour (2015), and the Eras Tour (2023–2024). After a 2019 dispute over the ownership of Swift's back catalog, she re-recorded the song as "I Knew You Were Trouble (Taylor's Version)" for her 2021 album Red (Taylor's Version).