The News of Cumberland County reports the child's mother briefly left him unattended in the tub with the water running and returned moments later to find him underwater.
Police did not release the identity of either the baby or the mother.
Police were called to the Delsea Gardens apartment complex shortly after 10 a.m. Monday for a report of a child in cardiac arrest.
Efforts to revive the baby were unsuccessful, and he was declared dead at South Jersey Healthcare Regional Medical Center.
