| Saint-Michel-de-Castelnau | |
| Commune Status: | Commune |
| Arrondissement: | Langon |
| Canton: | Le Sud-Gironde |
| Insee: | 33450 |
| Postal Code: | 33840 |
| Mayor: | Michel Darroman[1] |
| Term: | 2020 - 2026 |
| Intercommunality: | Bazadais |
| Coordinates: | 44.2822°N -0.1097°W |
| Elevation M: | 99 |
| Elevation Min M: | 78 |
| Elevation Max M: | 147 |
| Area Km2: | 42.61 |
Saint-Michel-de-Castelnau (in French pronounced as /sɛ̃ miʃɛl də kastɛlno/; Occitan (post 1500);: Sent Miquèu de Castèthnau) is a commune in the Gironde department in Nouvelle-Aquitaine in southwestern France.