"Baby" is a song by Canadian singer Justin Bieber, featuring American rapper Ludacris. It was released as the lead single on Bieber's debut album, My World 2.0. The track was written by Bieber, Ludacris, Christina Milian, Tricky Stewart (who worked with Bieber on a previous single "One Time"), and R&B singer The-Dream, with production helmed by the latter two.

It was available for digital download on January 18, 2010. The song received airplay directly after release, officially impacting mainstream and rhythmic radio on January 26, 2010. The song received positive reviews from critics who complimented the song's effective lyrics and chorus and commended Ludacris' part and the song's ability to have an urban twist. It is widely regarded as one of the defining songs of modern-day popular music and continues to find audiences globally.