The boy reportedly climbed onto a table and pressed up against the screen of an open window that gave way around 3:20 p.m. in the 100 block of a South Boulevard apartment, San Mateo police Sgt. Hugh Wilkins said.
He is being treated for severe head trauma, a punctured lung and bleeding in the brain, Wilkins said.
Lt. Michael Brunicardi said at least one of the parents was home at the time.
"This is just one of those tragic, horrific incidents that every parent would just shutter at if it happened to them." Brunicardi said.
A similar situation occurred in Fremont where a 3-year-old fell from a third-story window onto asphalt and was transported to a hospital on a Life Flight, according to Fremont Police Department Sgt. Chris Mazzone. The child is in good condition and is expected to survive.