The Women's 400 Individual Medley (or "I.M.") swimming event at the 2009 SEA Games was held in December 2009.
The Games Record at the start of the event was 4:51.87, swum by Joscelin Yeo at the 1999 SEA Games (on August 13).
| Place | Swimmer | Nation | Time | Notes |
|---|---|---|---|---|
| 1 | Natthanan J. | 4:55.07 | ||
| 2 | QUAH Ting Wen | 4:56.32 | ||
| 3 | Hui Yu Koh | 4:56.73 | ||
| 4 | Fibriani Marita | 4:57.16 | ||
| 5 | Chavisa T. | 4:58.30 | ||
| 6 | SIOW Yi Ting | 5:05.45 | ||
| 7 | T Kim Tuyen Nguyen | 5:05.45 | ||
| 8 | Ressa Ressa | 5:17.78 | ||
| Rank | Swimmer | Nation | Time | Notes |
|---|---|---|---|---|
| 1 | Fibriani Marita | 5:09.82 | Q | |
| 2 | SIOW Yi Ting | 5:11.21 | Q | |
| 3 | Chavisa T. | 5:13.84 | Q | |
| 4 | Natthanan J. | 5:18.71 | Q | |
| 5 | Ressa Ressa | 5:19.18 | Q | |
| 6 | QUAH Ting Wen | 5:19.06 | Q | |
| 7 | Hui Yu Koh | 5:20.33 | Q | |
| 8 | T Kim Tuyen Nguyen | 5:27.06 | Q | |
| 9 | Wei Li Lai | 5:27.23 |