"One Time" is the debut single of Canadian recording artist Justin Bieber. It is the first single released from his debut EP, My World. It was first solicited to mainstream and rhythmic radio on May 18, 2009. It was released via digital download in the United States and Canada on July 7, 2009, and was released in several other countries during autumn 2009. An acoustic version of the song, called the "My Heart Edition" was released to iTunes on December 22, 2009. The song, moderately paced with R&B, pop, and hip-hop influences, is an ode to young love.

The song received positive reviews from critics, commending its production, vocals and quality of lyrics. The song was a commercial success, reaching the top twenty in Canada, US, Germany, United Kingdom, France and New Zealand and also charted in several other countries. The single has been certified Platinum in Canada and the United States. The accompanying music video portrays Bieber at a house party, and features his mentor, Usher and hometown friend, Ryan Butler. As of July 2017, the video had been viewed over 530 million times on the video-sharing website YouTube. Bieber performed the song in a number of live appearances including The Dome, The Next Star, and Dick Clark's New Year's Rockin' Eve with Ryan Seacrest.

The music video, directed by Vashtie Kola, was posted by Bieber on his YouTube channel on June 13, 2009, almost a month before the single was released to iTunes. Bieber's mentor, Usher and one of his closest friends, Ryan Butler make appearances in the video.