Police say a large tree branch fell on the boy's arm, severing it.
It happened at approximately 6 p.m. at a home on Chandler Road in Cheltenham Township.
When first responders arrived at the home, the toddler's arm was trapped under the fallen branch.
Authorities say the boy was attending a birthday party when the incident took place.
He was flown to the Children's Hospital of Philadelphia for treatment. There is no word yet on his condition.