GENEVA — The World Trade Organization ruled Friday that China had violated trade rules when it slapped punitive duties as high as 21 percent on American-made cars and sport utility vehicles.

The ruling was an enormous victory for U.S. automotive manufacturers, coming at a time of rising tensions between the U.S. and China on a range of strategic issues. It was the second time in two months that the U.S. has won a WTO trade case against China. In March, the WTO ruled that China had restricted trade by imposing export duties on so-called rare earth elements, the lightweight, superconductive minerals that are critical to a wide range of high-tech products.

Michael Froman, the U.S. trade representative, hailed Friday’s ruling, which will have the effect of cutting the cost of U.S.-made cars and SUVs imported for sale inside China.

The U.S. auto industry employs nearly 850,000 workers throughout the country, from Michigan to California.

“The message is clear: China must follow the rules, just like other WTO members,” Froman said. He promised to continue to press China to change trade practices “that unfairly restrict U.S. exports.”

The duties — fees that an importer must pay on items it wishes to sell inside China — were imposed in 2011 in response to a 2009 petition from Chinese auto manufacturers that U.S. automobiles with large engines were being sold at unfairly low prices.