The boy, who was found unresponsive in the tub Wednesday night, died Friday morning, East Tennessee Children's Hospital spokeswoman Erica Estep told news outlets.
The boy's mother, Lindsee Leonardo, 32, was charged Thursday with aggravated child abuse, the Knox County Sheriff's Office said.
Leonardo told police she left Aiden Leonardo and a 1-year-old sibling in the tub so she could have some “me time,” an arrest warrant said. She told police she was away for about 10 minutes during which she smoked a cigarette and listened to music, the warrant states.
When Leonardo returned, she told police the infant was floating on his back, not breathing and she called 911. The toddler wasn't injured.
Sheriff's spokeswoman Kimberly Glenn says additional charges are expected.
It wasn't immediately clear whether Leonardo has an attorney.