The girl's babysitter, Humberto Hernandez-Pineda, 27, has been charged with felony murder.
Police said they received an injury call at 11:57 p.m. Friday from an apartment complex in the 1000 block of Franklin Road.
Officials found the girl, identified as Isabelle Cordero, suffering from a life-threatening head injury.
Police said Cordero was in the care of Hernandez-Pineda at the time.
Cordero succumbed to her injuries Monday, police said.
Hernandez-Pineda is being held at the Cobb County Adult Detention Center.
The case remains under investigation. Anyone with information is asked to call the Marietta Police Department at 770-794-5300 or Detective Cowell at 770-794-2363.