A compliance panel issued a report last week suggesting the United States failed to comply with a dispute settlement body's earlier recommendations and rulings, while unfairly restricting online casinos.
The panel stated that the U.S. ban on Internet gambling is acceptable only if restrictions are placed equally on domestic practices, including off-track betting on horse races.
The islands of Antigua and Barbuda sought the ruling from the WTO, saying the U.S. online gambling ban has hurt its economy.
The Caribbean country successfully argued that the United States is violating trade agreements by making exceptions for off-track betting.
Some have argued that the U.S. banned Internet gambling, not to protect Americans but to promote real-world casinos in places like Las Vegas.
U.S. Rep. Barney Frank, a Massachusetts Democrat, has said he is considering legislation to repeal the ban.
The ban makes it illegal for U.S. banks and credit card companies to process gambling payments.