"Thank You Girl" is a song recorded by the Beatles, written by John Lennon and Paul McCartney (Lennon–McCartney), and issued as the B-side of the single "From Me to You", which was recorded on the same day (5 March 1963). While not released on an LP in the United Kingdom until Rarities in 1978, the song was the second track on The Beatles' Second Album in the United States. As the B-side of the single "Do You Want to Know a Secret", it hit No. 35 on the Billboard Hot 100 in the spring of 1964.

Originally titled "Thank You, Little Girl", the song was written by John Lennon and Paul McCartney as a tribute to the band's many female fans. McCartney said, "We knew that if we wrote a song called, 'Thank You Girl', that a lot of the girls who wrote us fan letters would take it as a genuine 'thank you'. So a lot of our songs were directly addressed to the fans." Written “eyeball to eyeball", a phrase Lennon and McCartney would later use to describe their early formulaic writing sessions, "Thank You Girl" demonstrates how they were able to produce a song from scratch by working in total partnership. Lyrically, Ian MacDonald suggests that Lennon probably wrote the first line of each verse, allowing McCartney to use his flair for word play and inner-rhyming in completing it.

Lennon said the song was originally intended as a single: "'Thank You Girl' was one of our efforts at writing a single that didn't work. So it became a B-side or an album track." In April 1972, he told Hit Parader, "[The song was written by] Paul and me. This was just a silly song we knocked off." McCartney seemed to agree describing it as "a bit of a hack song, but all good practice."

Both "From Me to You" and "Thank You Girl" were credited to "McCartney–Lennon", as were eight of the songs on the Please Please Me album. It would be permanently changed to the more familiar "Lennon–McCartney" songwriting credit for their next single release, "She Loves You".

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