The ninth season of the American fictional drama television series ER first aired on September 26, 2002, and concluded on May 15, 2003. The ninth season consists of 22 episodes.

For the first time John Carter becomes the central character and Noah Wyle receives star billing. The death of Mark Greene continues to affect his colleagues while a grieving Corday has left Chicago for England. She returns and a medical student raises eyebrows. The ER is still plagued by the smallpox disease at the beginning. Elsewhere Romano suffers a horrific injury which has consequences throughout the season, Weaver finds herself promoted, Abby's family troubles resurface, Pratt continues to get on the wrong side of his colleagues, and Kovač and Carter join a relief mission in Africa, setting up a continuing story thread for following seasons. Carter deals with professional and family issues while other staff members have their own problems. Over the course of this season, Romano suffers setbacks after losing his arm, Abby and Carter lean towards a relationship, Pratt has troubles in both his personal and professional life.

Original executive producers John Wells and Michael Crichton reprised their roles. Long-time crew member Jack Orman returned as executive producer and show runner. Previous executive producer Christopher Chulack remained a consulting producer while working on Wells' Third Watch. R. Scott Gemmill and Dee Johnson continued to act as co-executive producers. Medical expert Joe Sachs remained a supervising producer. Richard Thorpe, Joe Sachs, and Wendy Spence Rosato returned as producers. They were joined by new producer Bruce Miller. Eighth season executive story editor David Zabel and unit production manager Tommy Burns joined the production team as co-producers for the ninth season. New crew member Julie Hébert began the season as a co-producer. Zabel and Hebert were promoted to producers mid-season. Hebert left the crew with the close of the season. Teresa Salamunovich returned to the crew as an associate producer for the ninth season. She was joined by new associate producers Erin Mitchell (for the entire season) and Shelagh O'Brien (after the mid-season break).

Wells wrote a further episode for the season. Gemmill was the season's most prolific writer with five episodes. Johnson and Orman each wrote four episodes. Zabel and Hebert each wrote three episodes. Sachs and Miller each wrote two episodes. Yahlin Chang joined the writing staff as a story editor in 2002 and contributed to four episodes as a writer and twelve episodes as an executive story editor and one episode as a co-producer between 2002 and 2005. Wells was promoted to executive story editor mid-season but left the staff with the close of the season. New writer Arthur Albert wrote a single episode.

Producers Kaplan and Thorpe served as the season's regular directors. Kaplan helmed five episodes while Thorpe directed four. Chulack directed a further episode. Show runner Orman helmed a further two episodes. New producer Hebert directed a single episode. Returning director Charles Haid directed two episodes. Cast members Laura Innes and Paul McCrane each directed an episode, McCrane making his series debut. Returning directors were Félix Enríquez Alcalá, David Nutter, Nelson McCormick, TR Babu Subramaniam, and Alan J. Levi. Peggy Rajski was the season's only new director.