Trump announced the cut on Monday which would leave 25,000 American troops in the European country, accusing Berlin of taking advantage of the US on trade too.
The reduction, which was first reported on June 5, would be a remarkable rebuke to one of the closest US trading partners.
It was unclear whether the plan would actually come to pass as some of the president's fellow Republicans in Congress have criticized it, arguing a cut would be a gift to Russia.
"So we're protecting Germany and they're delinquent. That doesn't make sense. So I said, we're going to bring down the count to 25,000 soldiers," Trump said, adding that "they treat us very badly on trade" without providing any details.
The US president went on to say that he would stick with the plan unless Germany changed course.