Police say the attack took place some time after 10 p.m. in the 700 block of Central Street, close to Northwestern University's campus and just about a block from the home of the university's president.
The victim was treated for minor injuries at a local hospital and released.
"All we have is a report that came through a third party person of a sexual assault occurring in the 700 block of Central Street, occurring last night between 10 p.m. and midnight," said Commander Jay Parrott, Evanston Police Department.
Police say the victim, who is a dog walker, was in Evanston for work. After the alleged attack, she notified a friend who called Evanston police. Investigators say the alleged victim told them the suspect displayed a handgun before sexually assaulting her outside.
"Right now our detectives have been working diligently to check all the information that has been provided by the victim and also the third party that also provided the information," said Parrott.
The victim provided police with only a general description of the suspect. Police are hoping that perhaps witnesses may be able to come forward who might have seen the suspect in the neighborhood.