"Half of My Heart" is a song by American rock singer John Mayer featuring Taylor. It is the third single from Mayer's 2009 album, Battle Studies. Mayer was nominated for a Grammy Award for "Best Male Pop Vocal Performance" in 2011 for the song.

"Half of My Heart" features more of a pop rock sound than Mayer's previous music. His previous albums explored blues and acoustic rock, especially in his 2006 album Continuum. It also has a carefree, sing-along sound to it.

The lyrics of the song are sung by both Mayer and Taylor; Taylor sings with Mayer in the chorus and in the song's bridge. The lyrics are about the duo, with half of their hearts saying they should continue to love each other, although the other half says they are not each other's true love, and cannot keep loving each other.

Mayer tweeted about wanting to record the song with Taylor in March 2009: "Waking up to this song idea that won't leave my head. 3 days straight now. That means it's good enough to finish," he wrote. "It's called "Half of My Heart" and I want to sing it with Taylor Swift. She would make a killer Stevie Nicks in contrast to my Tom Petty of a song."

In an interview shown on television called "John Mayer: On the Record with Fuse" (which only aired on the Fuse channel), Mayer's interviewer asked why the singer chose Taylor to duet with, since she was only 19 years old, and Mayer was 32. He responded by saying that he was not putting her on the album just to sell more copies for die-hard fans of hers that might just get the album for that song.

A new version of the song was made which does not feature Taylor. It also features different instrumentation. An alternate edit of this version was set to the music video, and does feature Taylor, although the video does not. This version was released as the "Pop Mix" on iTunes and Amazon.