The girl died early Wednesday morning, police said, a little more than a week after the assault.
The suspect, Adrian Morgan Allen, 27, of both Gooseneck Road in Bristol Township and Camden, N.J. faces first-degree felony aggravated assault and child endangerment charges.
There was no word on additional charges in light of the girl's death. Bensalem police said a news conference about the case is planned for next week.
Police say Allen was babysitting his girlfriend's three children at her apartment in the 2500 block of Knights Road in Bensalem on the weekend of February 24th to 26th.
It was on that Sunday when Allen allegedly called his girlfriend to say that her daughter was sick and vomiting.
The girlfriend returned home, police say, to find the child unresponsive in her crib. The mom took the baby to St. Mary Medical Center and the child was then flown to St. Christopher's Hospital for Children.
The exact circumstances of the assault were not disclosed by police.
Anyone with information is asked to call the Bensalem Police Special Victims Unit at 215-633-3658.
Allen was last reported to be in Bucks County Prison on 10% of $1 million bail.