Medics responded around 7 p.m. Thursday to the 1800 block of East Somerset Street.
The 1-year-old child was taken to St. Christopher's Hospital for Children, where she was pronounced dead at 7:49 p.m.
Authorities said the death is being investigated as a possible drowning.
Philadelphia Police Chief Inspector Scott Small said officers found a basin filled with water in a second-floor bathroom. Investigators also reported water covering much of the dining room area on the first floor.
"We're not really certain what happened at this time," Small said. "So it appears to be a possible drowning. Our Special Victims Unit is going to conduct an investigation."
Police said a search warrant is being obtained to process the home, which is standard procedure.
The circumstances surrounding the incident remain under investigation.