The incident occurred just before 10 p.m. in the 4800 block of Pine Street.
Police said a man came home to find his wife, 40-year-old Jennifer Joseph, with stab wounds to her wrist, which authorities believe were self-inflicted.
The man then found his 5-year-old son, Devon Joseph, lying face down in a tub that was filled with water. The boy had a laceration in his neck.
Medics performed CPR on the boy and rushed him to Children's Hospital of Philadelphia, but he was pronounced dead a short time later.
Authorities believe Jennifer has some mental health issues. She was taken to the hospital in stable condition and was conscious and talking.
Polly Scheryer, who works nearby, said she saw the police cars outside.
"You never know who is sick, because they look just like me and you," she said.
There is no word on whether authorities have been called to the home before.
"We will do a premise history of this particular location to see how often and when we've been to this location, if any, and the nature of the assignments but that's part of our routine investigation," said Philadelphia Police Chief Inspector Scott Small.
Said said Wednesday that Jennifer Joseph has been arrested and charged with Murder.
Police said the boy's father is cooperating with the investigation.