Allen auditioned for the eighth season of American Idol in Louisville, Kentucky with his brother Daniel, who didn't make it to Hollywood.He performed "I Want You Back," by The Jackson 5, during Hollywood week as part of a group named White Chocolate, which included fellow finalist and semi-finals roommate Matt Giraud. At the Idol Mansion, Allen was roommates with contestant Adam Lambert.According to the Us Weekly article about the Top 13, Allen stated, in regard to Lambert being a good fit, "We're both neat."
Allen was voted into the finals along with Lambert and Allison Iraheta on February 26, 2009. He has received many compliments throughout the competition for his folk-inspired interpretations of modern pop songs, and has played several instruments during his performances, including the acoustic guitar, the electric guitar, the keyboard, and the piano. Randy Jackson has compared him to Jason Mraz.His performance of "To Make You Feel My Love" as part of the Top 11 earned praise from the judges, with Simon Cowell commenting "I am genuinely beginning to think you have a shot of doing well in this competition."On the Top 9 show, he performed his own arrangement of "Ain't No Sunshine" on a keyboard, with a few musicians on stage, which earned praise from the judges. Cowell lauded it his "best performance so far."On the first Top 7 night, he chose to sing the Oscar-winning song from the indie film Once. While Jackson described it as "pitchy from note one",Kara DioGuardi declared it "one of your best moments ever."Due to time constraints, Paula Abdul and Cowell did not comment. But on the following night's results show, Cowell stated "Kris, you were brilliant."On the second Top 7 show, Allen brought back the guitar for the fourth time and performed an original, acoustic version of "She Works Hard for the Money", complete with bongo drums and the band on stage. This earned praise from all four judges, with Abdul remarking "There aren’t many men who are willing to shop in the women’s department. You shopped and found a perfect fit." For his second Top 3 performance, Allen impressed the judges with an acoustic arrangement of Kanye West's "Heartless", prompting Jackson to state that he preferred it to West's original and The Fray's cover version. Simon Cowell agreed and said, "I had written you out of the competition but that has changed after that performance."
Season 7 American Idol winner David Cook stated that Allen and Lambert are his favorites of Season 8. Country music singer and actress Reba McEntire has echoed the same opinion.
After being voted into the Top 3 Finalists, Kris Allen returned home to Arkansas and his hometown to film the material for the next 2 American Idol episodes. Allen was greeted by a crowd estimated to be greater than 20,000.Allen performed concerts in Little Rock and Conway, playing many of his popular songs during the competition, including "Man in the Mirror", "Ain't No Sunshine", "Falling Slowly", and "She Works Hard for the Money" at his venues; as well as "Come Together" at the local Fox News studio. He performed all of these songs with his guitar.
On May 20, 2009, Allen was declared the winner of American Idol, season 8, becoming the first married contestant to claim the title. During the finale, he performed with country artist Keith Urban on Urban's single, "Kiss a Girl". He also performed "We Are the Champions" by Queen as a duet with runner-up Adam Lambert.
