One of the Boys is the second studio album by American singer Katy Perry. It was released on June 17, 2008, by Capitol Records. She collaborated on the album with producers Greg Wells, Dr. Luke, Dave Stewart, Max Martin, and Ted Bruner among others, and wrote most of the songs in collaboration with other producers and writers. However, the title track, "Thinking of You", and "Mannequin" were all written by Perry alone. The EP Ur So Gay, containing the song of the same name, was released in 2007 to generate interest in the singer and the album.

One of the Boys is Perry's first album under the stage name Katy Perry, following her debut Katy Hudson (2001), which was released under her real name on Red Hill Records. One of the Boys received mixed reception from critics, with some deeming it as being filled with "potential hits" and others criticizing the material they perceived to be weak. Commercially, it debuted at number nine on the US Billboard 200, selling over 47,000 copies in its first week of sales, and also peaked within the top-ten charts in Austria, Canada, Denmark, Germany, Ireland, Mexico, Norway, and Switzerland. One of the Boys earned Perry two Grammy Award nominations, and has sold 7 million copies worldwide, being her third highest-selling album to date. It has been certified triple platinum by the Recording Industry Association of America (RIAA).

The album's lead single "I Kissed a Girl" became Perry's first Billboard Hot 100 number-one single; the second single "Hot n Cold" reached number three in the United States, while topping the charts in Germany, Canada, the Netherlands, and Austria, among others. The third single "Thinking of You" reached the top-thirty in the US, while the fourth and final single "Waking Up in Vegas" became a top-ten hit and her third top-ten hit overall. To further promote the album, Perry embarked on three concert tours: the 2008 Warped Tour, her first solo world tour, the Hello Katy Tour, and served as an opening act in No Doubt's 2009 Summer Tour.