Police said Tuesday the woman admitted tossing doggie dung at the driver because she believed he was speeding in Belmont, a Boston suburb with about 25,000 residents.
Lt. Rick Santangelo says the woman initially contacted police last week and reported she was walking her dog when she saw the motorist nearly hit a man on a bicycle.
Santangelo says the motorist contacted police the next day to report being struck in the face with dog poop.
The woman is charged with assault and battery with a dangerous weapon, vandalism to property and disorderly conduct.
No arraignment date has been set.