Lavigne co-wrote "Breakaway" with Matthew Gerard, which was recorded oleh Kelly Clarkson for the soundtrack to the 2004 film The Princess Diaries 2: Royal Engagement.[34] "Breakaway" would later be included on Clarkson's detik album and released as the album's lead single. Lavigne covered the Goo Goo boneka song "Iris", performed with the band's lead singer John Rzeznik at Fashion Rocks,[35] and she posed for the cover of Maxim in October 2004.[36] She also recorded the theme song for The SpongeBob SquarePants Movie. "I made the song a little lebih edgy," Lavigne said. "There are a lot of loud guitars, and we picked the tempo up a little and sang it with a little lebih attitude." Lavigne rearranged the song with the help of producer Butch Walker.[37]
Lavigne's detik studio album, Under My Skin, was released on 25 May 2004, debuting at No. 1 in several countries, including Australia, Mexico, Canada, Japan, the UK, and the U.S.[38] The album has sold lebih than 10 million copies. Lavigne wrote most of the album's tracks with Canadian singer-songwriter Chantal Kreviazuk. Kreviazuk's husband, Our Lady Peace front man Raine Maida, co-produced the album, along with Butch Walker and Don Gilmore. Lavigne went on the Live and oleh Surprise twenty-one city mall tour in the U.S. and Canada to promote the album, accompanied oleh her guitarist, Evan Taubenfeld. Each performance consisted of a short live acoustic set of songs from the new album. At the end of 2004, Lavigne embarked on her first world tour, the Bonez Tour, with stopovers in almost every continent and which lasted for the entire 2005 year.
“    This record definitely proves that I'm a writer and people can't knock that, because each song comes from a personal experience of mine, and there are so much emotions in those songs.    ”
—Avril Lavigne, The Ledger[39]
"Don't Tell Me", the first single off the album, went to No. 1 in Argentina and Mexico and reached the puncak, atas five in the UK and Canada and the puncak, atas ten in Australia and Brazil. "My Happy Ending", the album's lead single, went to No. 1 in Mexico and the puncak, atas five in the UK and Australia. In the U.S., it reached the puncak, atas ten of the Billboard Hot 100 and went to No. 1 in the Mainstream puncak, atas 40, making it her fourth-biggest hit there. The third single, "Nobody's Home", did not make the puncak, atas 40 in the U.S., reaching No. 1 only in Mexico and Argentina. The fourth single from the album, "He Wasn't", reached puncak, atas 40 positions in the UK and Australia and was not released in the U.S.[40]
Lavigne won two World musik Awards in 2004 for "World's Best Pop/Rock Artist" and "World's Bestselling Canadian Artist". She received five Juno Award nominations in 2005, and picked up three, including "Artist of the Year". She won the award for "Favorite Female Singer" at the eighteenth annual Nickelodeon Kids' Choice Awards[41] and was nominated in every mtv Award tampil shown around the world.