Her father, who was also wounded, remains in serious but stable condition.
Jonylah Watkins was shot five times Monday but succumbed to her wounds Tuesday after undergoing surgery at Comer Children's Hospital.
Witnesses say the girl's father, Jonathan Watkins, was standing beside his minivan on the south side of Chicago changing Jonylah's diapers when the gunman opened fire.
Jonylah was shot in the thigh, shoulder, lung, liver and bowels. Her father was shot in the side and buttocks, and suffered a graze wound to his face.
Police say Jonathan Watkins is a reputed gang member with a long criminal history. The baby girl's mother, Judy Watkins, was shot in the knee while she was pregnant with Jonylah.
The suspects involved in the shooting remain on the loose. Authorities are reportedly looking for security footage that might help them track down the shooter and the driver of the getaway van.
No information on whether the slain girl's father will cooperate in the investigation has been released.
The Associated Press contributed to this report.