Underclass Hero is Sum 41's fifth overall studio album and the first to be released without former guitarist Dave Baksh. The album was first released July 18, 2007 in Japan. It was released under the Island Records label and distributed worldwide by Universal Records, and in Canada by Aquarius Records. The album was released in Australia under Mercury Records. The title's similarity to John Lennon's hit "Working Class Hero" is not a coincidence, according to the band's front man Deryck Whibley, who in an interview with Sun Media claimed Lennon as his favorite songwriter.

Underclass Hero is over ten minutes longer than any previous Sum 41 album. This is a result of there being a total of fourteen tracks (plus one hidden track) compared to the set of twelve or thirteen the band has used before. Mostly however, the length is due to a majority of the songs being much longer than the usual three minutes existing on preceding full-lengths.

"I had to decide what I wanted to say with my music," explains Whibley. "I asked myself all these questions and then just pulled up my own answers and started writing songs based on those themes. I wanted to make an album that meant something important from beginning to end. I wanted it to have relevance and significance. It's not a concept album. It's not about fictional characters in a made-up story but there is a constant idea that runs through the record. It's a deeply personal statement that reflects the confusion and frustration in modern society."

The album has spawned three singles, "Underclass Hero", "Walking Disaster", and "With Me". Another song called "March of the Dogs" has also been released as an album preview. "Speak of the Devil" and "Count Your Last Blessings" were leaked onto the internet on July 13 2007. The entire CD was leaked onto file-sharing networks on July 14 2007.