According to the Calvert County Sheriff's Department, deputies were dispatched to Prancer Court in Lusby after the attack was reported just after 1 p.m.
Deputies arrived within one minute of the initial call, the department said, and saw the dog still attacking the child. They shot the dog to stop the attack and immediately determined that the boy had died.
The attack reportedly occurred while a family friend was watching the child.
An investigation is ongoing, and a news conference with Sheriff Mike Evans is scheduled for noon Friday.
Comment: Other recent reports of serious dog attacks within the last 3 days: Child taken to hospital following dog attack in El Paso County, Colorado
Pit bull in NJ 'viciously' bites 5 people, including child
North Carolina woman critically hurt after being attacked by great danes