The ninth season of American Idol premiered on Fox on Tuesday, January 12, 2010, and concluded on Wednesday, May 26, 2010. Simon Cowell, Kara DioGuardi, and Randy Jackson returned as judges and were joined by Ellen DeGeneres, who was brought on as a replacement for Paula Abdul after a series of guest judges filled in during the auditions. Idol Gives Back also returned on April 21, 2010. The top 24 semifinal format used in the fourth through seventh seasons also returned this season. Cowell, DioGuardi, and DeGeneres all left the show after this season, although only Cowell's departure was announced in advance. Lee DeWyze, a folk rocker from Mount Prospect, Illinois, won the competition, beating out fellow folk rocker Crystal Bowersox, who was the runner-up. Multiple contestants from this season were signed to record deals, including DeWyze, Bowersox, Casey James, Michael Lynche, and Siobhan Magnus.

Changes from previous seasons

Paula Abdul did not return to the judges panel, having announced in the beginning of August 2009 that she was leaving the show due to unresolved contract negotiations. Victoria Beckham, Mary J. Blige, Shania Twain, Katy Perry, Avril Lavigne, Joe Jonas, Neil Patrick Harris, and Kristin Chenoweth were brought on as guest judges during the auditions. After making the decision to utilize guest judges in Abdul's absence, Fox chairman Peter Rice stated that they would find a permanent fourth judge before the season premiere in January 2010. Ellen DeGeneres confirmed on September 9, 2009, that she would be joining the show as the new permanent fourth judge for the rounds held at CBS Television City for Hollywood Week and thereafter. Additionally, Simon Cowell announced shortly before the ninth season began that it would be his last season on American Idol. Cowell left the show to begin preparations for an American version of his singing competition The X Factor. Ellen DeGeneres also announced on July 29, 2010, that she would not return to the judges panel because it was not the "right fit" for her. Kara DioGuardi also announced on September 3, 2010, that she would not return as a judge for the tenth season.

Regional auditions

Auditions began on June 14, 2009, less than a month after the previous season's finale. During this stage, a series of guest judges filled in on the judging panel. Auditions were held in the following cities.

Hollywood week

Held at the Kodak Theatre for the second straight year, the first day of Hollywood Week featured the 181 contestants from the auditions round singing solo with the option of playing an instrument. Ellen DeGeneres made her first appearance as a judge at this time. 96 contestants advanced. The next round required the contestants to split into groups and perform. 71 advanced to the final round of Hollywood requiring a solo performance. 46 made it to the final round, where the judges spoke to the contestants one-by-one to tell them whether they had made the final 24.

Semifinals

The semifinal format used during the fourth through seventh seasons was revived for this season. Starting with twelve women and twelve men, the women and men performed weekly on separate shows, each performing a Billboard Hot 100 song, and on the results show, the bottom two contestants from each group were eliminated from the competition. The semifinals took place over three weeks, meaning that six men and six women were eliminated, leaving the other six men and six women to form the top 12. The women performed on the first night, and the males the next night. However, the men and women switched on March 2 and March 3, because Crystal Bowersox had been hospitalized due to complications from diabetes. Color key:

Top 24

Contestants are listed in the order they performed.

Top 20

Contestants are listed in the order they performed.

Top 16

Contestants are listed in the order they performed.

Top 12 finalists

Finals

There were eleven weeks of finals with twelve contestants competing. At least one contestant was eliminated every week based on the public's votes, although the judges could veto one elimination through the use of the "judges' save." Color key:

Top 12 – The Rolling Stones

Contestants performed one song each from the Rolling Stones discography, and are listed in the order they performed.

Top 11 – Billboard number ones

Miley Cyrus served as a guest mentor this week. Contestants performed one song each from the list of Billboard number one hits, and are listed in the order they performed.

Top 10 – R&B/Soul

Usher served as a guest mentor this week. Contestants are listed in the order they performed.

Top 9 (April 6th) – Lennon–McCartney

Contestants chose songs from the Lennon–McCartney discography, and are listed in the order they performed. The judges chose to use their "judges' save" when Michael Lynche was announced as the performer to be eliminated. As a result, no one was eliminated this week.

Top 9 (April 13th) – Elvis Presley

Adam Lambert served as a guest mentor this week. Contestants performed one song each from the Elvis Presley discography, and are listed in the order they performed. Two contestants were eliminated.

Top 7 – Inspirational music

Alicia Keys served as a guest mentor this week. Contestants are listed in the order they performed.

Top 6 – Shania Twain

Shania Twain served as a guest mentor this week. Contestants performed one song each from her discography, and are listed in the order they performed.

Top 5 – Frank Sinatra

Harry Connick, Jr. served as a guest mentor this week. Contestants performed one song each from the Frank Sinatra discography, and are listed in the order they performed.

Top 4 – Movie soundtracks

Jamie Foxx served as a guest mentor this week. Each contestant performed two songs: one solo and one duet with a fellow contestant. Contestants are listed in the order they performed.

Top 3 — Contestant's Choice/Judges' Choice

Each contestant performed two songs: one chosen by the judges and one chosen by the contestant. Contestants are listed in the order they performed.

Top 2 – Finale

Each contestant performed three songs, one of which was chosen by producer Simon Fuller, and are listed in the order they performed.

Elimination chart

Color key: • {{legend|gold|Winner}} • {{legend|silver|Runner-up}} • {{legend|#FDFC8F|Saved by the public}} • {{legend|#B0E0E6|Saved by the judges}} • {{legend|#e0f0ff|Bottom two or three}} • {{legend|#FF91A4|Eliminated}}

Controversies

Chris Golightly was originally selected as semi-finalist. According to reports, Golightly was disqualified on February 17, 2010, after already being told he was in the top 24, over an old contract. The contract had expired by the time the top 24 began to tape, but they disqualified him because he was under contract at the time of the tryouts, in violation of Idol rules. He was later replaced by Tim Urban at the last minute of the last part of Hollywood Week. Ryan Seacrest was criticized by TheWrap and MSNBC Entertainment for "critiquing the judges’ comments" and becoming more aggressive. Ratings for this season dropped significantly compared to the previous two seasons, leading to speculation that it might be coming to a close. It was speculated that reasons for the decline might have included the pending departure of Simon Cowell and the more unpredictable behavior of Ryan Seacrest.

Season nine semifinals shock eliminations

Michael Slezak of EW stated, "So you’d think by season 9 — during the inconsequential semifinals, no less — it would be easy to brush off the bad judgment of speed-texting tweens, to process the way-too-early exits of Lilly Scott, Katelyn Epperly, and Alex Lambert (plus the sort-of-maybe-too-early ouster of Todrick Hall), and move on. So why do the ritual killings of the dreams of four indisputably flawed semifinalists feel like they’re part of something bigger, a shifting of the tectonic plates at the core of the Idol universe?"

U.S. Nielsen ratings

This season of American Idol was the top show for the 2009–10 season of broadcast primetime shows. Its Tuesday and Wednesday episodes occupied the top two spots of the season. Viewership for the Tuesday episodes averaged 22.974 million, while the Wednesday episodes averaged 21.951 million.