The comments by Richard Grenell, the U.S. ambassador to Germany, signal Trump's impatience with Merkel's failure to raise defense spending to 2% of economic output as mandated by the NATO military alliance.
"It is offensive to assume that the U.S. taxpayers continue to pay for more than 50,000 Americans in Germany but the Germans get to spend their (budget) surplus on domestic programs," Grenell told the dpa news agency.
Germany's fiscal plans foresee the defense budget of NATO's second-largest member rising to 1.37% of output next year before falling to 1.24% in 2023.
Eastern European countries like Poland and Latvia, fearful of Russia after it annexed Crimea from Ukraine in 2014, have raised their military spending to the 2% target, drawing praise from Trump who wants Germany to do the same.
U.S. complaints about Germany's defense spending pre-date Trump but relations with the United States have deteriorated since he became president.
The two allies do not see eye-to-eye on a range of issues, including Iran, trade tariffs and the NordStream 2 gas pipeline from Russia to Germany.
Trump said in June he would deploy 1,000 U.S. troops from Germany to Poland, which sees the measure as deterrence against possible aggression from Russia.
Georgette Mosbacher, U.S. ambassador to Poland, has made a similar criticism of Germany's reluctance to commit more financial resources to NATO.
"Poland meets its 2% of GDP spending obligation towards NATO. Germany does not. We would welcome American troops in Germany to come to Poland," she wrote on Twitter on Thursday.
The United States has more than 33,000 soldiers in Germany and an additional 17,000 U.S. civilian employees to support them. It is believed the United States also has nuclear warheads in Germany.
"President Trump is right and Georgette Mosbacher is right," Grenell told dpa. "Multiple presidents have asked Europe's largest economy to pay for its own defense. This request has been made over many years and by many presidents."
Grenell added that the United States must react if Germany continues to ignore Trump's demand to boost defense spending.
Trump travels to France this month for the G7 summit where Iran will be a major topic. Trump will also visit Poland and Denmark.Grenell earlier this month criticized Germany for showing reluctance to join a planned U.S. naval mission in the Strait of Hormuz, close to Iran.