"Mr. Spaceman" is a song by the American rock band The Byrds and was the third track on their 1966 album Fifth Dimension. The song was initially written by band member Jim McGuinn as a "melodramatic screenplay" but it soon evolved into a whimsical meditation on the existence of extraterrestrial life. After its appearance on Fifth Dimension, "Mr. Spaceman" was released as the third single taken from that album in September 1966 (see 1966 in music). The single reached #36 on the Billboard Hot 100 but failed to chart in the United Kingdom.

Written in early 1966 by McGuinn, the song was—along with "5D (Fifth Dimension)"—one of two science fiction-themed songs on the Fifth Dimension album. Upon its release as a single, the music press coined the term "space-rock" to describe the song, although since then, the term has come to refer to a genre of rock music originating from 1970s progressive and psychedelic music. Musically, "Mr. Spaceman" has a country-style backing, albeit with touches of psychedelia, and can clearly be seen as a precursor to the band's later exploration of country music on Sweetheart of the Rodeo. As such, the song is often cited as being one of the earliest examples of the country rock genre. The title also recalled that of The Byrds' earlier worldwide smash hit and debut single, "Mr. Tambourine Man".

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