The victim was attacked in a deserted walkway which runs beneath the Customs roundabout, near Stirling.
She was in the south side of the area between 4.30am and 5am near Wallace Street when her assailant pounced.
The scene is in a residential area less than a mile east of the Stirling University campus.
All pedestrian routes to and from the roundabout were closed off yesterday as investigations got under way. The area was cordoned off with police tape as officers remained at the scene.
The area is near Orchard House hospital and a handful of traders, including a hairdresser's and a roof repairs business.
The roundabout, known locally as the Clock Tower roundabout, joins the A9 from Bridge of Allan to Bannockburn with the city and is one of the busiest routes in the city during daytime.
A spokesman said: "We are investigating following a report of a serious sexual assault on a 19-year-old woman in the underpass.
"Inquiries into the incident, which happened sometime between 4.30am and 5am on Monday, are ongoing, and anyone with information that can assist police with their investigation is asked to come forward immediately."