A viewer sent Action News images of smoke pouring from the Adelphia House Apartments on the 1200 block of Chestnut Street.
The fire started in a 4th-floor apartment, according to firefighters.
One resident was rushed to Jefferson Hospital with smoke inhalation, and is in serious but stable condition.
Neighbors said they smelled smoke and then helped each other get out safely.
It took crews about an hour and a half to get the fire under control.
The cause is under investigation.
The Red Cross is assisting.
Fire crews are checking for any structural damage.