The Flagler County Sheriff's Office said 41-year-old James Irvine of Palm Coast was arrested early Saturday.
The child's mother called police after learning Irvine left the baby alone while he went out to drink, one newspaper reported. She was at work at the time.
A police report said Irvine told the mother the "pit bull was watching the baby."
The child was in his bedroom with the door shut when she came home. A deputy reported the dog was sitting outside the room.
It's not known whether Irvine has an attorney.