A 19-year-old woman was seriously sexually assaulted within a vehicle in the area of Yeaman Shore opposite Dundee Railway Station on Friday.
The woman then ran from the area and a man who was walking a dog came to her assistance.
Detective Inspector Sarah Brow said: “This has been a terrifying attack on the woman and she is currently receiving support from our officers.
“We are keen to speak to the man who was walking his dog who helped her and anyone else who was in the area at the time as any piece of information, no matter how small, can help.
"We have extra evening patrols in the area and anyone with any concerns should get in touch.”
Anyone with any information should contact Police via 101 quoting reference number 0431 of Friday, 27 August.