The tourist attraction has been a target lately. This is the second time this month and the third time in eight months it has been cleaned up after an incident.
On September 4, someone poured white paint all over it.
The rock was spray-painted red back in February and police later charged a 17-year-old.
Police said there are security cameras in the area.
Plymouth Rock marks the 1620 voyage of the Mayflower to North America and is known symbolically as the “Landing Place of the Pilgrims.”