The WTO says its appellate panel issued the ruling confidentially to the two parties Wednesday. It is likely to be made public in the coming weeks.
The EU and the U.S. last year appealed the decision of a WTO panel that found Boeing had received more than $5 billion in prohibited U.S. subsidies to develop and build new planes. The EU says the illegal subsidies were far higher.
The case mirrors a U.S. complaint Washington brought over European aid to Boeing rival Airbus.
U.S. trade officials say Brussels has so far failed to comply with that panel's order to slash support for Airbus.