The accident happened shortly after 2:30 p.m. in the area of Aramingo and West Moreland in the Port Richmond section.
The 7-week-old baby was thrown some 10 feet after being hit.
The baby was taken to St. Christopher's Hospital for Children and is now in stable condition.
Police say the male male driver is in custody, and was tested for DUI. Charges are pending the results of the tests.
There is no word yet on how the crash happened.