Police and medics were called to the scene, an apartment building in downtown Tacoma, just after 4 a.m. Saturday, said Tacoma Fire Battalion Chief Lincoln Correa.
Arriving at the residence, they found a 2-year-old boy who was having difficulty breathing. He was rushed to the hospital, where he died shortly afterward.
A 19-year-old man who said he was the boy's father was arrested at the scene and booked into Pierce County Jail.
Charges are expected to be filed against him on Monday, and an autopsy is scheduled to determine exactly how the boy died.
Tacoma police spokeswoman Loretta Cool said it appears there was a "pattern of abuse" toward the child.
The boy who died was the only child in the home. The mother reportedly was out of town, Cool said.
The names of the boy and the suspect were not immediately released.