The father was holding the two children when a wave swept all three into the water at about 12:30 p.m. Saturday in the Falcon Cove area, near the small coastal community of Cannon Beach, authorities said.
Jeremy Stiles, 47, of Portland and his daughter were taken by ambulance to a hospital in the nearby city of Seaside, while the Coast Guard deployed helicopters to help search for the boy.
A police officer initially found the man struggling to get out of the water, while the girl was farther offshore. She was pronounced dead at the hospital.
The father’s condition was not immediately available.
The hours-long search for the boy was suspended once it became dark.
The National Weather Service had issued a high-surf warning in the area for Saturday, as large waves made for dangerous surf topping 30 feet (9.1 meters).
On Sunday, the weather service also issued a beach hazard update that warned of sneaker waves that could suddenly pull people into the water and be strong enough to lift and roll logs.