"Who's That Girl" is a song by American singer-songwriter Madonna from the soundtrack album Who's That Girlof 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 Hot 100, while peaking atop the charts in countries like the United Kingdom, Canada, Netherlands, Ireland and Belgium.

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. It was criticised for not depicting her real self. 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.

In 1986, Madonna was shooting for her third motion picture Who's That Girl, known at the time as Slammer. Needing songs for the soundtrack of the movie, she contacted Patrick Leonard and Stephen Bray, who had written and produced her third studio album True Bluein 1986. Madonna explained to them that she needed an uptempo song and a downtempo song. She came to the studio one Thursday as Leonard developed the chorus of the song. He handed over that cassette to Madonna, who went to the backroom and finished the melody and the lyrics of the song, while Leonard worked on the other parts of the song. After finishing the lyrics, Madonna declared that she wanted the song to be named "Who's That Girl" and changed the movie to the same, rather than Slammer, considering it to be a better title. In Fred Bronson's The Billboard Book of Number 1 Hitsbook, Leonard explained that the song was recorded in one day with Madonna adding her vocals only once. Additional instrumental tracks with guitars and percussion were included by Leonard and Bray later. Regarding the development of the music for the film, Madonna further explained

"I had some very specific ideas in mind, music that would stand on its own as well as support and enhance what was happening on screen and the only way to make that a reality was to have a hand in writing the tunes myself. [...] The songs aren't necessarily about Nikki [her character name in the movie] or written to be sung by someone like her, but there's a spirit to this music that captures both what the film and the characters are about, I think."

The song is composed in Madonna's typical style—mingling the drum machine, a bubbling bass synth line, and the sound of stringed instruments. The three parts of the song, namely the bridge, where Madonna sings "what can help me now", the chorus and the verse flow together strongly. The chorus has a haunting effect in it. The song epitomizes Madonna's interest with Hispanic culture that continued after the release of "La Isla Bonita", by adding Spanish phrases in the chorus and over the trumpets of the second verse, and also by the added instrumental break in the middle. It also uses the sonic effect brought about by the combination of multiple vocal lines, which had been previously used by groups like The Beach Boys in their singles "God Only Knows" (1966) and "I Get Around" (1964) as well as R.E.M.'s singles "Fall On Me" (1986) and "Near Wild Heaven" (1991). "Who's That Girl" employs this effect on the last chorus where three or four different vocal hooks are intertwined.

Critical reaction to the song has been mixed. In his book The Complete Guide to the Music of Madonna, author Rikky Rooksby explained that the song was Madonna's best take on her original music style. Stephen Thomas Erlewine of Allmusic commented that "Who's That Girl" along with "Causing a Commotion" were not amongst Madonna's best singles. Although Tony Power of Blendermagazine criticised the whole soundtrack, he considered "Who's That Girl" to be its standout track. Biographer J. Randy Taraborrelli, in his book Madonna: An Intimate Biographycalled the song "quintessential Madonna music" and went on to describe it as "funky, sassy and melodic, with a Latin accent." Noah Robischon from Entertainment Weeklyopined that with the both the song and the movie, Madonna had pushed "synergy over the borderline." Bill Lamb of About.com said that the song, along with the second single from the soundtrack "Causing a Commotion", was not an example of Madonna's best music.

"Who's That Girl" was released in the United States in June 1987. It debuted on the Hot 100 at number forty-three, reached the top of the chart in its seventh week, maintained the top position for one week, and spent sixteen 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 two 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 eighty-third position of the RPMsingles chart on July 11, 1987, reached the top for one week on August 29, 1987, and stayed on the chart for twenty-three weeks. 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. 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.

The music video was shot over two days, at Grayhall Mansion in Beverly Hills, California. Madonna had adopted a garish, platinum blond hairstyle for the Who's That Girlmovie which she used in the music video; it was her way of reviving the comedy heroine of the 1930s Hollywood screwball comedies. She continued with her Hispanic look from the "La Isla Bonita" music video, this time she appeared dressed boyishly in a wide-brimmed Spanish hat and bolero jacket—a combination which would later become a fashion trend.

The music video, directed by Peter Rosenthal, begins with Madonna entering a park. After meeting two children and a teenage boy, they start roaming around the park, with Madonna singing the song. These scenes are interchanged with scenes from the motion picture, which show Madonna as the movie character Nikki Fynn. As the music video progresses, Madonna is shown to be in search of an Egyptian treasure casket. After being directed to it by a postcard displaying her cartoon impersonation, Madonna opens it to find a huge diamond. She looks up happily to the children. The video ends with them continuing dancing and Madonna carrying away the casket.

The video portrayed a different image of Madonna rather than her real self. According to Vincent Canby of The New York Times, Madonna at that time was shrewdly pragmatic about her persona and appearance—resembling Marilyn Monroe, but with the "comic tartiness" of Jean Harlow. This persona was reflected in the second half of the Who's That Girlfilm. However, the music video chose not to capture her real self and qualities, or to promote the movie for which it was specifically created. Instead, it concentrated on the humerous off-putting personality of Madonna's film character depicted in the first half of the film. As a result, the video failed to appeal to people who were not acquainted with her prior work, nor with people who already knew and admired Madonna's erotic and funny performance in her music videos.

Madonna performed the song live only once, on her 1987 Who's That Girl World Tour. The song was performed as the part of the encore. The outfit for the performance, designed by Marlene Stewart, was created with toy watches, plastic lobsters and an ashtray—thereby depicting her awareness of surrealism. According to biographer Andrew Morton, it also portrayed her tongue-in-cheek sense of style: when she bent over in this ensemble, she revealed panties with the word "Kiss" written on them. The performance included Madonna dancing around the stage in the dress and asking the audience to join her. Two different performances of the song on this tour can be found on the videos: Who's That Girl - Live in Japan, filmed in Tokyo, Japan, on June 22, 1987, and Ciao, Italia! - Live from Italy, filmed in Turin, Italy, on September 4, 1987.

The song has been covered many times, mostly on tribute albums. In 1999, The Countdown Singers made a sound-alike cover version for the album Hit Parade of 80's, Vol. 2. The Royal Philharmonic Orchestra (RPO) made an instrumental version of the song for the 1998 album Material Girl: RPO Plays Music of Madonna. German actress Eva Mattes covered the song for her 2006 album Language of Love. In 2007, The Bubonic Plague recorded a cover of the song that was included on the tribute album Through the Wilderness.