The incident occurred around 2:00 p.m. Thursday at 18th and Walnut streets.
The fire department says a 30-year-old male window washer fell from the seventh floor to the second floor roof of the Rittenhouse Claridge.
A co-worker said the man's harness rope broke.
Witness Brian Height said he was playing chess when the worker fell.
"I just heard a noise, then I saw someone sort of rappel down the side of the building and telling someone to call 911," said witness Brian Height.
He watched as firefighters used ladders to get to the injured worker. The man was then rushed Hahnemann Hospital where he was last reported to be in stable condition.
"Someone did say they saw him wave his hand or move his hand while he was in the gurney, so that's a good sign," Height said.
L&I has been called to the scene to inspect the window washing equipment.