See main article: Swimming at the 2009 SEA Games.
The Men's 400 Individual Medley swimming event at the 25th SEA Games was held on December 10, 2009. This even was swum as a timed-final (i.e. swimmers only swam the race once).
| Place | Swimmer | Nation | Time | Notes |
|---|---|---|---|---|
| 1 | 4:27.00 | |||
| 2 | 4:27.57 | |||
| 3 | 4:30.78 | |||
| 4 | 4:32.44 | |||
| 5 | 4:34.24 | |||
| 6 | 4:35.39 | |||
| 7 | 4:39.00 | |||
| 8 | 4:44.97 | |||
| 9 | 4:48.29 | |||
| 10 | Nithipanya A | 5:28.92 | ||
| - | DQ | |||