The FAA's finding paves the way for commercial air service at the county-owned airport; however, the final decision will be made by the Snohomish County Council.
An environmental review in September found that adding some commercial flights would not significantly increase noise, traffic or air pollution.
Allegiant Air wants to start operating four flights a week.
Alaska Air Group spokeswoman Bobbie Egan says it prefers Sea-Tac, but if a competitor started using Paine Field it would be compelled to add flights there, too, on Horizon Air or Alaska Airlines planes.
Paine Field already handles wide body jets from the nearby Boeing factory, but neighbors have objected to regular commercial service.