Officers were called at around 12.40am to reports of a single vehicle collision involving a Ford Fiesta that had left the carriageway.
An occupant of the Ford Fiesta, a 21-year-old woman from Fareham, died as a result of the collision.
Her next of kin have been informed and are being supported by specialist officers.
Officers are investigating the exact circumstances of the collision and are keen to speak to anyone with information.
Officers are particularly keen to speak to the driver of a silver Ford Transit van who we believe may have seen the Fiesta travelling along the road prior to the collision and then came across the crash scene.
Do you drive a silver Ford Transit van and were you on Downend Road this morning? Maybe a friend or family member drives one?
If you have any information, or dash cam footage, please get in touch on 101 quoting Operation Sedge or 44200230693.