Swimming at the 2005 World Aquatics Championships – Men's 400 metre individual medley

The Men's 400m Individual Medley (or "I.M.") event at the 2005 FINA World Aquatics Championships was swum on 31 July 2005 in Montreal, Quebec, Canada. Preliminary heats were held in the day's morning session, with the top-8 finishers advancing to swim again in the event's final that evening.

Prior to the competition, the existing World (WR) and Championship (CR) records were as follows.
 * WR: 4:08.26 swum by Michael Phelps (USA) on 14 August 2004 in Athens, Greece.
 * CR: 4:09.09 swum by Michael Phelps (USA) on 27 July 2003 in Barcelona, Spain