The ruling from a WTO panel was the latest blow in the decade-long clash between the titans of the civil aircraft industry, which has seen both Airbus and Boeing score points along the way.
The WTO ruled in March 2012 that billions of dollars of subsidies to Boeing were illegal and notified the United States to bring them to an end.
But just a few months later, the European Union filed a new complaint with the global trade body, alledging that Washington was not complying with that order.
It agreed with Brussels that the United States had not taken "appropriate steps to remove the adverse effects or ... withdraw the subsidy."
More From This Section
The EU hailed the ruling, saying in a statement that it confirmed "that the United States has taken no appropriate action to comply with its obligation to withdraw subsidies declared illegal ... Or to remove their adverse effects."
"The panel agrees that the US has simply ignored existing WTO rulings and has continued to subsidise Boeing," EU Trade Commissioner Cecilia Malmstrom said in the statement.
Washington therefore chose to see the ruling as a victory, pointing out that the WTO panel had found that 28 of the 29 programmes it examined were in compliance.
"The panel found only one state-level programme ... To be contrary to WTO rules," the US government said in a statement.
"The United States disagrees and plans to appeal this limited finding," it added.
In September, a WTO panel found that Brussels had not respected a 2011 ruling ordering it to take steps to withdraw several support and subsidy programmes for Airbus.
The WTO did not put a value on those programmes but Boeing said they amounted to $22 billion worth of illegal support for Airbus development and sales.
The EU has appealed against that decision, insisting that the subsidies in question had been discontinued.
US Trade Representative Robert Lighthizer meanwhile insisted that Friday's ruling proved that "the United States does not provide subsidies even remotely comparable to the uniquely large and uniquely harmful EU subsidies to Airbus."