Kutai, who had been suffering from an illness for about a month, died Saturday afternoon, the (Portland) Oregonian reported.
Zoo officials said the Sumatran orangutan was responding to treatment, but his health deteriorated Saturday and he died after undergoing surgery.
A necropsy was performed Saturday night to determine why the animal's health declined so rapidly, officials said.
Kutai was born at Sedgwick County Zoo in Kansas and moved to the Oregon Zoo in 2001. His grandmother, Inji, 54, is the oldest animal at the zoo in Portland.