"Who's That Girl" is a song by American singer-songwriter Madonna from the soundtrack album Who's That Girl of the motion picture of same name. It was released on June 30, 1987, by Sire Records as the first single from the album. It later appeared on the 1991 UK compilation EP The Holiday Collection, which was released to accompany the 1990 compilation album The Immaculate Collection, and has since been included on the two-disc edition of her 2009 greatest hits album Celebration. While shooting for the film, then called Slammer, Madonna had requested Patrick Leonard to develop an uptempo song that captured the nature of her film persona. She later added the lyrics and vocals to the demo tape developed by Leonard, and decided to call both the song and the movie "Who's That Girl".

Featuring instrumentation from drums, bass, and stringed instruments, "Who's That Girl" continued Madonna's fascination with Hispanic culture by incorporating Spanish lyrics and using the effect of double vocals. Although it received mixed reactions from reviewers, the song became Madonna's sixth single to top the Billboard Hot 100, while peaking atop the charts in countries like the United Kingdom, Canada, Netherlands, Ireland and Belgium. It was also nominated for "Best Song From A Motion Picture" at the 1988 Grammy Awards and "Best Original Song" at the 1988 Golden Globe Awards.

The music video portrayed a different persona of Madonna, rather than her film character for which it was released. Like the song, it incorporated Hispanic culture and portrayed her dressed in Spanish style as a young lady in search of a treasure. Madonna performed the song once on the Who's That Girl World Tour in 1987. The song has been covered by many artists and has appeared in compilations and tribute albums.

"Who's That Girl" was released in the United States in June 1987. It debuted on the Billboard Hot 100 at number 43, reached the top of the chart in its seventh week, maintained the top position for one week, and spent 16 weeks on the chart. It became Madonna's sixth number-one single in the United States, making her the first artist to accumulate six number-one singles in the 1980s, and the first female performer to get that many number-ones as a solo act. The song peaked at number forty-four on the Hot Dance Club Play chart. In 2000, the song came tenth in a vote conducted to determine the favourite Madonna song. In Canada, the song debuted at the 83rd position of the RPM singles chart on July 11, 1987, reached the top for one week on August 29, 1987, and stayed on the chart for 23 weeks.It placed at 12 on the RPM Year-end chart for 1987.

In the United Kingdom, "Who's That Girl" was released on July 14, 1987, and debuted at number three on the UK Singles Chart, soon climbing to number one the next week to become Madonna's fifth number-one single in the United Kingdom. According to The Official Charts Company, the song has sold 380,000 copies there. Across Europe, "Who's That Girl" also topped the singles charts in Belgium, Italy, Ireland, and the Netherlands, as well as peaking in the top five in Austria, France, Germany, Norway, Sweden, and Switzerland.It was certified gold by the Syndicat National de l'Édition Phonographique for shipment of 500,000 copies in France.