| Image Coa: | DEU Unterdießen COA.svg |
| Coordinates: | 47.9833°N 60°W |
| Image Plan: | Unterdießen in LL.svg |
| State: | Bayern |
| Region: | Oberbayern |
| District: | Landsberg am Lech |
| Verwaltungsgemeinschaft: | Fuchstal |
| Elevation: | 610-644 |
| Area: | 12.79 |
| Postal Code: | 86944 |
| Area Code: | 08243 |
| Licence: | LL |
| Gemeindeschlüssel: | 09 1 81 143 |
| Divisions: | 3 Ortsteile |
| Website: | www.unterdiessen.de |
| Mayor: | Alexander Enthofer[1] |
| Leader Term: | 2020 - 26 |
Unterdießen is a municipality in the district of Landsberg in Bavaria in Germany. It is located just west of the river Lech.