The victim had just crossed Park Place while walking south on Moore Street during the Tuesday evening incident, police said.
The woman was standing on the sidewalk on the southwest corner of the intersection of Moore Street and Park Place when a man approached her from behind and grabbed her buttocks just before 6:15 p.m., Princeton Police Lt. Christopher Tash said in a release.
The man then ran west on Park Place, Tash said.
The man is described as Hispanic, in his 30s, around 5 feet 4 inches tall with a medium to slightly heavy build, police said.
He was wearing a dark-colored beanie style winter hat, a white surgical mask and a navy blue jacket.
It was not immediately clear whether the incident is related to previous criminal sexual contacts in the area, police said.
Anyone with information is asked to contact Det. Don Mathews at (609) 921-2100 ext. 2137.
Click here to follow Daily Voice Hamilton Township and receive free news updates.