The World Trade Organisation says the US aircraft manufacturer, Boeing, received at least $US5.3 billion in unfair aid from Washington.
A panel of international trade judges in Geneva has ruled the subsidies included money for research and development from the NASA space agency.
Last year, the WTO said that Airbus received illegal aid from European governments.
The two companies have been at war over state aid for almost six years.
The case is one of the most complex ever brought before the WTO, which has issued 2000 pages of rulings. It recommends that the United States withdraw the subsidies.
