"I Will Possess Your Heart" is an alternative rock song recorded by the American band Death Cab for Cutie. The song depicts a one-sided obsessive relationship, which led Paste to name it one of the 25 creepiest songs about love. It is notable for its five-minute instrumental introduction influenced by krautrock, as well as its music video which required location shooting across four continents. The song was the lead single from their sixth studio album, Narrow Stairs (2008).

The music is dominated by a repetitive bass guitar riff interspersed with piano chords and Ben Gibbard's vocals. It was released in 2008 in two versions: a full eight-minute album version, and a four-minute radio edit that omitted most of the instrumental introduction. The song was critically acclaimed and nominated for the 2009 Grammy Award for Best Rock Song.

The music video, released on April 11, 2008, features scenes of a young woman traveling alone to various places around the world, interspersed with the band performing in an industrial freezer room. The shoot involved the actress, director, and a crew of two traveling 27,977 miles in 13 days, and was shot on consumer-grade camcorders to achieve a sense of total realism and so that the film crew would blend in as tourists. The video won the 2008 MTV Video Music Award for Best Editing and was nominated for the Best Cinematography award.

The song's lyrics depict a one-sided obsessive relationship from the point of view of the pursuer, who implores the object of his affection that she needs to "spend some time" with him so that he might "possess [her] heart." Songwriter and lead vocalist Ben Gibbard noted that, although fictional, the song was inspired by the experiences of some of his friends. He added:

The song is basically about a stalker. It’s about this nice guy who wants this girl he can’t have, and he believes they’ll be together once she realizes how great he is—he just has to wait it out. That’s the part that makes the song really creepy, the delusion of thinking that they were meant to be together. It’s a really dark song. A lot of the material is about the inevitable disappointment people feel as they move through life, and things don’t feel the way they expect. No experience will ever match up to the idealized version in your mind.

The song is written in the key of D, but the main motif continually switches modes from major to minor as the chord goes from D to F with a moderate tempo of 134 beats per minute. The music is dominated by a distinctive 4-bar bass guitar riff that is repeated through nearly all of the song. This is interspersed with piano chords that generally follow a D–D/C–F–G progression. The lead vocals have a range of D3–G4. The instrumentation also includes multiple guitar parts, drums, and backup vocals. Gibbard characterized the song as "five minutes of build and then a three-minute song", referring to the lengthy instrumental introduction that takes up more than half the song, influenced by the krautrock band Can. Like the other songs on Narrow Stairs, it was recorded with all band members playing together in the studio, directly to tape without any overdubs.

Gibbard credited Nick Harmer's bass line with being integral to the song, noting inspiration from bassists Eric Avery and Simon Gallup. Harmer said of the bass line, "I immediately gravitated to the creepy, stalker-ish theme that Ben created with the lyrics and the piano chords.... I liked the idea that once a stalker gets obsessed with an idea, it just keeps repeating in his head, so I wanted the bass line to have a repetitive, incessant theme."

The song was released on March 18, 2008, initially as a "surprise stream" on the band's website, and soon after was picked up by radio stations. It was released in two versions: the album version of the song is over eight minutes long, while the radio edit largely removes the instrumental introduction, shortening the song to four minutes.