| Event: | Men's 400m I.M. |
| Competition: | 2005 FINA World Aquatics Championships |
| Dates: | 31 July 2005 |
| Competitors: | 36 |
| Nations: | 28 |
| Win Value: | 4 minutes 09.63 seconds |
| Gold: | László Cseh |
| Goldnoc: | HUN |
| Silver: | Luca Marin |
| Silvernoc: | ITA |
| Bronze: | Oussama Mellouli |
| Bronzenoc: | TUN |
| Prev: | 2003 |
| Next: | 2007 |
See main article: Swimming at the 2005 World Aquatics Championships.
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.
| Place | Swimmer | Nation | Time | Notes |
|---|---|---|---|---|
| 1 |  Hungary | 4:09.63 | ||
| 2 |  Italy | 4:11.67 | ||
| 3 |  Tunisia | 4:13.47 | ||
| 4 | Italy | 4:13.48 | ||
| 5 |  USA | 4:13.67 | ||
| 6 |  Japan | 4:17.72 | ||
| 7 | USA | 4:17.93 | ||
| 8 | Greece | 4:19.12 | ||
| Rank | Heat+Lane | Swimmer | Nation | Time | Notes | |
|---|---|---|---|---|---|---|
| 1 | H5 L4 | László Cseh | 4:14.15 | q | ||
| 2 | H5 L5 | Luca Marin | 4:15.18 | q | ||
| 3 | H5 L6 | Robert Margalis | 4:16.25 | q | ||
| 4 | H4 L4 | Alessio Boggiatto | 4:16.46 | q | ||
| 5 | H4 L5 | Ioannis Drymonakos | 4:16.84 | q | ||
| 6 | H3 L4 | Oussama Mellouli | 4:17.21 | q | ||
| 7 | H3 L3 | Ryan Lochte | 4:17.26 | q | ||
| 8 | H5 L3 | Hidemasa Sano | 4:17.64 | q | ||
| 9 | H3 L6 | Nicolas Rostoucher | 4:20.00 | |||
| 10 | H4 L2 | Taishi Okude | 4:20.71 | |||
| 11 | H5 L7 | Robin van Aggele | 4:20.90 | |||
| 12 | H4 L6 | Igor Berezutsky | 4:20.98 | |||
| 13 | H3 L2 | Dean Kent | 4:21.04 | |||
| 14 | H4 L3 | Travis Nederpelt | 4:21.10 | |||
| 15 | H3 L5 | Ioannis Kokkodis | 4:23.82 | |||
| 16 | H3 L7 | Dmytro Nazarenko | 4:24.15 | |||
| 17 | H5 L2 | Adam Lucas | 4:24.24 | |||
| 18 | H2 L1 | Mihail Alexandrov | 4:25.50 | |||
| 19 | H3 L1 | Bradley Ally | 4:28.91 | |||
| 20 | H3 L8 | Paulius Andrijauskas | 4:29.25 | |||
| 21 | H2 L2 | Miguel Molina | 4:30.95 | |||
| 22 | H4 L1 | Sebastian Stoss | 4:31.25 | |||
| 23 | H2 L5 | Hocine Haciane | 4:31.33 | |||
| 24 | H2 L8 | Shaune Fraser | 4:31.90 | |||
| 25 | H1 L4 | Dmitriy Gordiyenko | 4:32.05 | |||
| 26 | H2 L4 | Guntars Deicmans | 4:34.00 | |||
| 27 | H2 L3 | Yury Zaharov | 4:39.22 | |||
| 28 | H1 L5 | Vasilii Danilov | 4:40.07 | |||
| 29 | H5 L8 | Sheng-hieh Tang | 4:41.75 | |||
| 30 | H4 L8 | Yu-An Lin | 4:44.06 | |||
| 31 | H2 L7 | Lionel Lee | 4:46.60 | |||
| 32 | H1 L3 | Francisco Montenegro | 4:49.14 | |||
| 33 | H1 L6 | Johnny Castillo | 4:52.21 | |||
| - | - | Mohamad Al-Naser | DNS | |||
| - | - | Keith Beavers | DNS | |||
| - | - | Sébastien Rouault | DNS | |||