San Antonio police spokesman Joe Rios identified the baby as Scott Wesley Buchholtz-Sanchez, born June 30.
Rios said the baby's mother 33-year-old Otty Sanchez, of San Antonio, will be charged with capital murder.
Otty Sanchez was taken to a local hospital Sunday with self-inflicted stab wounds to her chest and stomach.
A spokeswoman at University Hospital in San Antonio said she couldn't release information on Sanchez's condition, but Sgt. Wes McCourt said her wounds didn't appear to be life threatening.
Rios said Sanchez was sitting on the couch "screaming that she killed her baby" when police were called to the home around 5 a.m. Sunday.
"She mentioned that someone or something told her to do it, she was hearing voices. So that leads us to believe that she was experiencing some type of mental crisis," Rios said. "The baby had obviously been decapitated."
McCourt said she told investigators that the devil told her to kill her son.
The infant's body was found mutilated in a bedroom. He likely died earlier Sunday morning, police said.
A woman who answered the phone Sunday at the Bexar County Medical Examiner's office said she couldn't release information about the baby's cause of death.
Investigators took a sword, a machete and a kitchen knife from the home.
Officers also found two children unharmed at the home, though it wasn't immediately clear where they were taken.