It's unclear why, but the driver plunged into the black waters in Barnstable County just after 10:30 p.m. at the end of a dead-end road in West Yarmouth, Yarmouth police said.
When rescuers arrived, the car was about 100 feet from Bay Street Beach and fully submerged in water that was 15 feet deep, police continued. Witnesses told rescuers there was someone in the car, and they hadn't surfaced.
Yarmouth police, firefighters, and the Department of Natural Resources dove into the water and pulled the woman to safety. However, she was rushed to the hospital for treatment, police said.
Authorities did not provide any updates on her condition or her identity.
Foul play is not suspected.
Click here to follow Daily Voice Westport and receive free news updates.