The attack happened Saturday around 2 p.m. near Fair Oaks Avenue and Green Street.
Police say the woman was with her friend trying to catch a bus and get lunch. However, witnesses say a man approached the victim and punched her without warning.
First responders found the victim conscious but unresponsive. She remains hospitalized in stable condition, but she suffered serious injuries.
"This was totally unprovoked," Pasadena police Lt. Monica Cuellar said about the assault.
Michael Vigil, 40, was taken into custody for the assault. He was arrested on three felony counts, including felony battery and elder abuse. His bail was set at just over $1 million.
Police say Vigil is currently unhoused.
Police give credit to the community for helping officers locate Vigil in a matter of minutes, not far from where the assault took place.
People in the area say they are disturbed by the attack.
"It's already hard enough being a woman, walking alone at night, but to know that there's actively assaults happening so close to me, it makes me want to be more aware," said Nyah Toomes, who works in the area.
Others expressed concern about the homeless issue.
"I think it's an unusual occurrence," Pasadena resident Nan Merrit said. "This is not something that's happening every day, thank heavens. It makes me feel really bad about those who are unhoused in Pasadena. We have a big problem."