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