Wakefield District CID would like to speak with the woman who assisted the distressed victim in Ackworth Road, Featherstone, in the early hours of Wednesday.
Officers believe the lady spoke with and comforted the teenage girl, who she found sat on the street outside a house.
The lady spoke with the girl and left the scene after confirming that she was not in any imminent danger.
The teenager later contacted police to report she had been the victim of a serious sexual assault, police said.
A man has been arrested in connection with the assault.
Detective Sergeant Phil Davies of Wakefield District CID, said: “A number of enquiries are ongoing into this offence, and I am appealing to the woman who we believe provided comfort to the victim to come forward as a potential witness.
“We know the lady pulled up in her car after seeing the girl at the roadside and stopped to comfort her, and believe she was not made aware of what had taken place."
He added the police "do need to hear from her as part of our investigation".
She is asked to call Wakefield District CID on 101, referencing crime number 13180443858.