Athletics at the 2008 Summer Olympics – Men's 400 metres Semifinal 3 Semifinal 3

| Rank_ | Lane | Athlete | Nationality | Time_ | React |
|---|---|---|---|---|---|
| 1 | 7 | LaShawn Merritt | United States | 44.12 | 0.187 |
| 2 | 6 | Martyn Rooney | Great Britain | 44.60 | 0.126 |
| 3 | 8 | Johan Wissman | Sweden | 44.64 | 0.211 |
| 4 | 5 | Renny Quow | Trinidad and Tobago | 44.82 | 0.204 |
| 5 | 2 | Gary Kikaya | DR Congo | 44.94 | 0.187 |
| 6 | 9 | Michael Mathieu | Bahamas | 45.56 | 0.203 |
| 7 | 4 | Sean Wroe | Australia | 45.56 | 0.205 |
| 8 | 3 | Cedric van Branteghem | Belgium | 45.81 | 0.199 |