Police were called to the home for a welfare check after a friend said she hadn't heard from Micela Quinn, 40, who had medical problems. The friend said she hadn't been able to get in touch with Quinn, The (Portland) Oregonian said.
The boy was unhurt, though hungry and thirsty, Portland police Sgt. Pete Simpson said.
"It's truly heartbreaking," he said. "We can only assume what this poor kid was thinking about the past couple of days. Fortunately, the little guy is OK."
Police were working to notify next-of-kin though it didn't appear Quinn had any relatives in the area. The child was placed in protective custody while police worked to find family to take him in.