Youngsters are believed to have been climbing on the roofs of shops in Abington Street.
The injured boy did not suffer life threatening injuries and has been taken to Northampton General Hospital.
A Northamptonshire Police spokeswoman said: "We were called at 2.10pm to assist fire and ambulance following an incident of children on the roofs in Abington Street.
"The injuries are not thought to be life threatening or life changing at this time."
Darren Michael, who works at Pertemps in Abington Street, said: "We heard all the commotion and saw the air ambulance.
"It was all going off down the street.
"I just heard someone had gone through the roof.
"I wasn't sure whether it was through the roof or onto the street."
It is believed the boy fell onto one of the roof canopies and that there were several other boys with him at the time.
Earlier this year, two boys were filmed walking along rooftops in Abington Street.