Half of My Heart is a song by American rock singer John Mayer featuring American country pop singer Taylor Swift. It is the third single from Mayer's 2009 album, Battle Studies.
"Half of My Heart" debuted at 29 on the Billboard Hot Adult Top 40 Tracks chart on the issue date of May 8, 2009. This makes John Mayer's 14th appearance on that chart. The song also debuted at #25 on the Billboard Hot 100 upon the release of Mayer's album Battle Studies. It fell off the Hot 100 within two weeks, however upon its release as a single it re-entered the chart at #88 and reached #34.