The United States, European Union, Canada, Turkey and Mexico had complained about China restricting exports of the materials, since it had promised to eliminate almost all export duties and quotas when it joined the WTO a decade ago.
China had argued it was entitled to some exceptions under WTO rules.
"Today's report is a tremendous victory for the United States - particularly its manufacturers and workers," US Trade Representative Ron Kirk said in a statement.
"The Obama Administration will continue to ensure that China and every other country play by the rules so that US workers and companies can compete and succeed on a level playing field."
EU Trade Commissioner Karel De Gucht said the decision would force China to drop export restrictions for the materials in question, as well as for rare earths, which are subject of a separate case.
"This final ruling is a great success in our efforts to ensure fair access to the much needed raw materials for EU industry," he said in a statement.