Police are investigating reports of the serious sexual assault in Stirling between 4.30am and 5am.
The attack is said to have happened at Customs Roundabout, near Laurencecroft.
Officers have urged anyone with information to get in touch.
A Police Scotland spokesman said: "Police in Forth Valley are investigating following a report of a serious sexual assault on a 19-year-old woman in the underpass at Customs Roundabout, near Laurencecroft in Stirling.
"Inquiries into the incident, which happened sometime between 4.30am and 5am on Monday 5th May, are ongoing and anyone with information that can assist police with their investigation is asked to come forward immediately."