Under My Skin is the second studio album by Canadian singer-songwriter Avril Lavigne. It was released on May 19, 2004, by Arista Records and RCA Records. Lavigne wrote most of the album with singer-songwriter Chantal Kreviazuk, who invited her to a Malibu in-house recording studio shared by Kreviazuk and her husband Raine Maida, where Lavigne recorded many of the songs. The album was produced by Maida, Don Gilmore, and Butch Walker. It is Lavigne's final album to be released on Arista, as she departed the label after its release.

Under My Skin debuted at number one on the Canadian Albums Chart and on the US Billboard 200. It sold three million copies in the United States, ranking the album number 149 on the Billboard 200 decade-end chart. Because of the album's darker, heavier, more aggressive vibe reminiscent of post-grunge and more melodic rockier songs, it received generally positive reception from critics at the point of considering it as one of the classic albums that defined pop punk music in the early 00's, and also one of the works that anticipated the emotional intensity and theatrical aesthetics of emo pop music in the mainstream. On March 18, 2013, Under My Skin was re-released as a double-disc set paired with her debut studio album, Let Go, which was released under Arista Records. To promote the album, Lavigne went on a promotional tour for malls in the United States and Canada. Furthermore, Lavigne embarked on a concert tour, entitled the Bonez Tour, starting on September 26, 2004, and ending one year later on September 26, 2005. The concert at the Budokan Stadium in Japan was filmed and released on a DVD only available in Japan, entitled Bonez Tour 2005: Live at Budokan. Under My Skin had sold 10 million copies worldwide and is the fifth best selling album of the 21st century by a Canadian artist.

Critics described Under My Skin as post-grunge, alternative rock, and nu metal containing some gothic rock, grunge, folk rock, and hard rock.