"If You're Not the One" is written as a love ballad. Bedingfield claims to have been inspired by the band Westlife, when writing and recording the song. Bedingfield himself believes that the song is "cheesy." Bedingfield admitted he began writing the song because he realized that material he considered more "complex", was unlikely to get him a record deal. "Commercialism, sappy lyrics and meek tunes are the things I hate most in the universe, but I'm not sure even Bob Dylan could get record company interest without hooks these days", Bedingfield recalled in an interview with The Daily Telegraph. "It's a different age. You need to go some kind of populist route. So, about three years ago, I sat down with a Westlife song and tried to write something similar."
Bedingfield also admitted that he didn't even want to put the song on his album. He said, "I didn't want to put "If You're Not the One" on the album as I thought it was too cheesy. I thought it sounded like Westlife. But she (Natasha Bedingfield, his sister) loved it. Women that hear it go all soft and think it's lovely"
"If You're Not the One" plays for 4 minutes and 19 seconds, in common time. It is in the key of B-flat major, and features the chorus features Bedingfield singing falsetto, where he hits the note E♭5.
"If You're Not the One" received mixed reviews from contemporary music critics. On the positive side, Peter Robinson, in the Gotta Get thru This album review for the New Musical Express (NME), wrote "If You're Not the One [recalls] every chart-mauling uber-ballad you've ever heard". Nedd Raggett of AllMusic commented in his album review "When it comes to ballads, they work best in sudden moments - the soaring end to If You're Not the One".About.com ranked the song at number 27 on its list of the 'Top 100 Pop Songs of 2002', two places behind the album's title track. The song was again ranked at number 27 on the 2003 list, due to it being released in the United States during that year.Music Week felt that the song showed Bedingfield's "sensitive side", and that the song "showcases his vocal range to stunning effect." The magazine even considered it "beautifully crafted."
However, the track also received some negative feedback. PopMatters' Nikki Tranter wrote "surely you would think with his obviously overwhelming ability to compose utter claptrap cleverly disguised as serious prophesizing, his lyrics might contain something a little more expressive than 'If you're not the one / Why does my heart feel glad today / If you're not the one / Then why does my hand fit yours this way.'" In his review for musicOMH, Michael Hubbard was even less positive, stating, "If You're Not the One is as vomit-inducing as anything the man in the plastic mask has ever wretched (sic) forth at the world".
When the song was released as a single, it was picked up by radio stations. It was the most added pop song on the radio for the week of 20 February 2003. In the US, the song was the most added song in the Adult Contemporary market for the week of 7 April 2003. In the US, the song was his highest debuting single, debuting at number 55 on the Billboard Hot 100. The single proved to be a successful release for Bedingfield, providing him with several worldwide number ones and reaching the top 20 in all but one of the charts that it appeared on. It debuted at number one in the UK Singles Chart, knocking Christina Aguilera's "Dirrty" from the top spot.
The music video was produced by A1 singer Mark Read. It begins with a series of images of Bedingfield singing in various positions against a black backdrop. When the chorus starts he is seen singing with clouds in the background. The initial sequence is repeated for the second verse. However, half-way through, he starts to write on a wall. The rest of the song follows the same pattern.