| Image Coa: | Wappen Grassau.svg |
| Coordinates: | 47.7789°N 12.45°W |
| Image Plan: | Grassau in TS.svg |
| State: | Bayern |
| Region: | Oberbayern |
| District: | Traunstein |
| Elevation: | 538 |
| Area: | 35.76 |
| Postal Code: | 83224 |
| Area Code: | 08641 |
| Licence: | TS |
| Gemeindeschlüssel: | 09 1 89 120 |
| Website: | www.grassau.de |
| Mayor: | Stefan Kattari[1] |
| Leader Term: | 2020 - 26 |
| Party: | SPD |
Grassau is a market town in the district of Traunstein in Bavaria in Germany. It is located south of lake Chiemsee in the valley of the Tiroler Ache.
Grassau consists of the districts of: