Girl power is a slogan that encourages and celebrates women's empowerment and positivity. The slogan's invention is credited to the American punk band Bikini Kill, who published a zine called Bikini Kill #2: Girl Power in 1991. It was then popularized in the mainstream by the British girl group Spice Girls in the mid-1990s. According to Rolling Stone magazine, the Spice Girls' usage of "girl power" was one of the defining cultural touchstones that shaped the Millennial generation, particularly during their childhood in the 1990s. The usage of the slang term is still considered relevant three decades later in the 2020s through continued feminist movements.

In 1990, American punk band Bikini Kill started to make their self-titled feminist zine. Its first issue had the subtitle, A color and activity book. A year later the band published the second issue of their Bikini Kill zine, with the new subtitle Girl Power. The band's lead singer, Kathleen Hanna, said it was inspired by the Black Power slogan. The authors of Young Femininity: Girlhood, Power and Social Change argue that the term also draws inspiration from 80s Black female, hip hop vernacular, "You go girl."

The term became popular in the early and mid 90s punk culture. The Rolling Stone Encyclopedia of Rock & Roll credits the zine with coining the slogan: "In their feminist fanzine Bikini Kill they articulated an agenda for young women in and outside of music; the band put those ideas to practice. Bikini Kill earned a reputation in the punk underground for confronting certain standards of that genre; for example, asking people to slam at the side of the stage, so that women would not get pushed out of the front, and inviting women to take the mic and talk about sexual abuse."

The phrase is sometimes sensationally spelled "grrrl power", based on the spelling of "riot grrrl." The Riot Grrrl movement was pioneered by Bikini Kill in the 1990s through their music and pushback on the punk industry’s sexist culture.

Some other musical artists who have used the slogan in their music are Welsh band Helen Love, with it appearing in the chorus of their 1992 song "Formula One Racing Girls", and pop-punk duo Shampoo, who released an album and single titled Girl Power in 1995. The slogan was integrated into children’s entertainment as well, The Cheetah Girls released their single "Girl Power" in 2003 through Disney.

British pop quintet Spice Girls brought the mantra into the mainstream consciousness in the mid-1990s. The Spice Girls' version of "girl power" focused on the importance of strong and loyal friendship among women and girls, with a message of empowerment that appealed to young girls, adolescents and adult women. According to Billboard magazine, they demonstrated real, noncompetitive female friendship, singing: "If you wannabe my lover, you gotta get with my friends. Make it last forever; friendship never ends."

In all, the focused, consistent presentation of "girl power" formed the centrepiece of their appeal as a band. Some commentators credit the Spice Girls with reinvigorating mainstream feminism in the 1990s, with the "girl power" mantra serving as a gateway to feminism for their young fans. On the other hand, some critics dismissed "girl power" as no more than a shallow marketing tactic, while others took issue with the emphasis on physical appearance, concerned about the potential impact on self-conscious and/or impressionable youngsters: As American feminist Jennifer Pozner famously remarked, it was "probably a fair assumption to say that a 'zig-a-zig-ah' is not Spice shorthand for 'subvert the dominant paradigm'". Regardless, the phrase became a cultural phenomenon, adopted as the mantra for millions of girls and even making it into the Oxford English Dictionary. In summation of the concept, author Ryan Dawson said, "The Spice Girls changed British culture enough for Girl Power to now seem completely unremarkable."

In 2018, Rolling Stone named the Spice Girls' brand of "girl power" on The Millennial 100, a list of 100 people, music, cultural touchstones and movements that have shaped the millennial generation.