It happened at 10:45 a.m. in the 500 block of East State Street.
Police say the man who was injured was inside the home when part of it came down.
The injured man suffered severe head trauma and was transported to Capital Health Regional Medical Center in Trenton.
His condition is not yet know.
Investigators say they don't know why the man was inside the dilapidated structure.
While the exact cause of the collapse is not yet known, investigators said that years of neglect combined with the rainfall could have been contributing factors.