Grass Flat is an unincorporated community and ghost town in northwestern Sierra County, California, United States.

Grass Flat is on Port Wine Ridge Road in the Sierra Nevada, within Plumas National Forest.

The site is 5.5 miles (8.9 km) southwest of Mount Fillmore.

The scarred landscapes from hydraulic mining during the gold rush, at Grass Flat and nearby, remain primarily barren of vegetation over 160 years later.

Ponds fill the depressions formed by the hydraulic surface mining technique.