Swimming at the 2009 World Aquatics Championships – Men's 4 × 100 metre medley relay
The men's 4×100 metre medley relay at the 2009 World Aquatics Championships took place on 2 August 2009 at the Foro Italico in Rome, Italy.
Records
The existing records when the event started were:| Record | Swimmer | Time | Location |
| World record | Aaron Peirsol Brendan Hansen Michael Phelps Jason Lezak | 3:29.34 |  Beijing 2008 |
| Championship record | Aaron Peirsol Brendan Hansen Ian Crocker Jason Lezak | 3:31.54 |  Barcelona 2003 |