"5D (Fifth Dimension)" is a song by the American rock band The Byrds, written by band member Jim McGuinn. It was released as a single on June 13, 1966 and reached #44 on the Billboard Hot 100 but failed to chart in the United Kingdom. The song was also included on the band's third album, Fifth Dimension, released on July 18, 1966. "5D (Fifth Dimension)" followed the release of the band's influential "Eight Miles High" single and arguably took The Byrds' psychedelic experimentation to even further extremes than their previous single had.