The blaze damaged a room at the Salman Al-Farisi Islamic Center in the pre-dawn hours as the Portland region remained abuzz about the aborted bomb plot at the city's annual Christmas tree lighting ceremony.
The (Portland) Oregonian said investigators were declining to say how the fire started.
The leader of the Islamic Center, Imam Yosof Wanly, said 19-year-old suspect Mohamed Osman Mohamud stopped by the mosque once or twice a month and was not considered a regular attendee.
Mohamud, a student at nearby Oregon State University, was arrested in an FBI sting operation as he allegedly tried to detonate a phony bomb supplied by the undercover agents.
"I know people here know the true reality of the Muslim community here," Wanly said. "It's a sad situation."