"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.

According to McGuinn, the song's lyrics were an attempt to explain Einstein's theory of relativity and were directly inspired by the book 1-2-3-4, More, More, More, More by Don Landis. In a 1966 interview with Hit Parader magazine the guitarist stated "It's sort of weird but...what I'm talking about is the whole universe, the fifth dimension, which is height, width, depth, time and something else. But there definitely are more dimensions than five. It's infinite. The fifth dimension is the threshold of scientific knowledge." Talking to Michael Ross of Creem magazine in 1970, McGuinn further explained the song's meaning: "'5D' was an ethereal trip into metaphysics, into an almost Moslem submission to an Allah, an almighty spirit, free-floating, the fifth dimension being the 'mesh' which Einstein theorized about. He proved theoretically - but I choose to believe it."

The song's abstract lyrics were largely interpreted by the band's audience as being about an LSD trip, much to McGuinn's dismay. The notion that the song was about drugs was given further credence when it was singled out, within a month of its release, by Variety magazine as one of a recent spate of pop songs containing references to illegal drug use. As a result of these allegations, the song was banned by some radio stations in the U.S.

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