Lavigne co-wrote "Breakaway" with Matthew Gerard, which was recorded 由 Kelly Clarkson for the soundtrack to the 2004 film The Princess Diaries 2: Royal Engagement.[34] "Breakaway" would later be included on Clarkson's 秒 album and released as the album's lead single. Lavigne covered the Goo Goo 玩偶 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 更多 edgy," Lavigne said. "There are a lot of loud guitars, and we picked the tempo up a little and sang it with a little 更多 attitude." Lavigne rearranged the song with the help of producer Butch Walker.[37]
Lavigne's 秒 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 更多 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 由 Surprise twenty-one city mall tour in the U.S. and Canada to promote the album, accompanied 由 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 最佳, 返回页首 five in the UK and Canada and the 最佳, 返回页首 ten in Australia and Brazil. "My Happy Ending", the album's lead single, went to No. 1 in Mexico and the 最佳, 返回页首 five in the UK and Australia. In the U.S., it reached the 最佳, 返回页首 ten of the Billboard Hot 100 and went to No. 1 in the Mainstream 最佳, 返回页首 40, making it her fourth-biggest hit there. The third single, "Nobody's Home", did not make the 最佳, 返回页首 40 in the U.S., reaching No. 1 only in Mexico and Argentina. The fourth single from the album, "He Wasn't", reached 最佳, 返回页首 40 positions in the UK and Australia and was not released in the U.S.[40]
Lavigne won two World 音乐 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 音乐电视 Award 显示 shown around the world.