Avon and Somerset Police are appealing to trace a woman who stopped to help a distressed teenage girl in the Priorswood area of Taunton at about 20:35 on Friday 4 September.
Officers believe the woman saw the girl being physically assaulted by a man, intervened and took her home.
Avon and Somerset Police said:
“A man, 22, has been arrested on suspicion of assault and remains in police custody at this time.
The victim is recovering at home and being supported by officers.”
Detectives want to hear from the woman who helped as soon as possible and ask her to call 101 quoting reference 5220201256.
Police would also like to hear from anyone with any other information, dashcam or CCTV footage which could help the investigation.