"...Baby One More Time" is the debut single by American singer Britney Spears from her debut studio album of the same name (1999). It was written by Max Martin and produced by Martin and Rami Yacoub. Released on September 29, 1998, by Jive Records, the song became a worldwide success, topping the charts in over 20 countries, including the United States and the United Kingdom, where it earned quintuple and triple-platinum certifications from the Recording Industry Association of America (RIAA) and the British Phonographic Industry (BPI), respectively, and was the latter's best-selling single of 1999. A teen pop and dance-pop song about longing for the return of an ex-boyfriend, "...Baby One More Time" is one of the best-selling singles of all time, with over 10 million copies sold.

An accompanying music video, directed by Nigel Dick, features Spears as a high-school student who starts to sing and dance around the school, while watching her love interest from afar. In 2010, the music video for "...Baby One More Time" was voted the third most influential video in the history of pop music, in a poll held by Jam!. In 2011, "...Baby One More Time" was voted by Billboard to be the best music video of the 1990s. It has been featured on all of her greatest hits and other compilation albums. In 2020, Rolling Stone named "...Baby One More Time" as the greatest debut single of all time. In 2021, the song was ranked at number 205 on the list of Rolling Stone's 500 Greatest Songs of All Time.

Spears has performed "...Baby One More Time" in a number of live appearances and during all of her concert tours. The song was nominated for Best Female Pop Vocal Performance at the 42nd Annual Grammy Awards (2000), and has been included in lists by Blender, Rolling Stone and VH1. It has been noted for redefining the sound of late 1990s music. Spears has named "...Baby One More Time" as one of her favorite songs from her catalog. It was also the final song to be played on the BBC's music programme Top of the Pops in the 1990s. A cover of the song by Windy Wagner was featured in the 2011 dance video game by Ubisoft, Just Dance 3. In 2018, readers of German teen magazine Bravo voted "...Baby One More Time" to be the biggest hit since its first music compilation was released in 1992.

In June 1997, Britney Spears was in talks with manager Lou Pearlman to join female pop group Innosense. Her mother Lynne Spears asked family friend and entertainment lawyer Larry Rudolph for his opinion and submitted a tape of Spears singing over a Whitney Houston karaoke song along with some pictures. Rudolph decided he wanted to pitch her to record labels, therefore she needed a professional demo. He sent Spears an unused song from Toni Braxton; she rehearsed for a week and recorded her vocals in a studio with a sound engineer. Spears traveled to New York with the demo and met with executives from four labels, returning to her hometown of Kentwood, Louisiana the same day. Three of the labels rejected her, arguing audiences wanted pop bands such as the Backstreet Boys and the Spice Girls, and "there wasn't going to be another Madonna, another Debbie Gibson, or another Tiffany." Two weeks later, executives from Jive Records returned calls to Rudolph. Senior vice president of A&R Jeff Fenster stated about Spears' audition that "It's very rare to hear someone that age who can deliver emotional content and commercial appeal. [...] For any artist, the motivation—the 'eye of the tiger'—is extremely important. And Britney had that." They appointed her to work with producer Eric Foster White for a month, who reportedly shaped her voice from "lower and less poppy" delivery to "distinctively, unmistakably Britney." After hearing the recorded material, president Clive Calder ordered a full album. Spears had originally envisioned "Sheryl Crow music, but younger more adult contemporary" but felt all right with her label's appointment of producers, since "It made more sense to go pop, because I can dance to it—it's more me."

I had been in studio for about six months listening and recording material, but I hadn't really heard a hit yet. When I started working with Max Martin in Sweden, he played the demo for 'Baby One More Time' for me, and I knew from the start it [was one] of those songs you want to hear again and again. It just felt really right. I went into the studio and did my own thing with it, trying to give it a little more attitude than the demo. In 10 days, I never even saw Sweden. We were so busy.

— Britney Spears, talking to Chuck Taylor, Billboard

Fenster asked producer Max Martin to meet Spears in New York, after which he returned to Sweden to write her a handful of songs with long-term collaborator Denniz Pop. Pop was ill, so Martin asked producer Rami Yacoub to help. When six songs were ready, Spears flew to Cheiron Studios in Stockholm, Sweden, where half of the album was recorded in May 1998, nominally produced by Martin, Pop and Yacoub. Pop, however, was too ill to attend any of the recording sessions, and Spears never met him. In his place, Martin was the acting producer.

Martin showed Spears and her management a track titled "Hit Me Baby One More Time", which was originally written for American boy band Backstreet Boys and the R&B girl group TLC, but they both rejected it. The label thought the song would work for the English group Five, but they also passed on it. Spears later said that she felt excited when she heard it and knew it was going to be a hit record. Jive A&R man Steve Lunt recalled, "We at Jive said, 'This is a fuckin' smash'"; but other executives were concerned that the line "Hit Me" would condone domestic violence. The title was revised to "...Baby One More Time".