Officers say the woman, who is in her 80s, had just walked into an apartment complex in the 200 block of 6th Avenue South Wednesday afternoon when a man punched her in the head.
The blow toppled the woman, and police say the attacker then jumped on top of her and sexually assaulted her at knife-point.
The attacker then stole the woman's money and ran out of the building, according to police.
"Our Investigation indicated he was milling around or hanging around that door just waiting sizing people up and looking for what victim he wanted," said Jeff Kappel with the Seattle Police Department.
The victim was taken to Harborview Medical Center to be treated for her injuries.
Police captured the suspect Thursday morning.
Based on security footage from the building, police believe the same man is responsible for a July 19 strong-arm robbery in the complex.