Steinmeier told Reuters in a written interview that he was concerned about what he called Trump's ambiguous vows to "make America strong again," while simultaneously reducing its engagement overseas.
"That is contradictory and it makes me concerned," Steinmeier said as he headed to Washington for meetings with other foreign and defense ministers to assess the US-led effort to defeat the Daesh (ISIL) terrorist group.
"A politics of fear and isolation will bring less security, not more, and would be dangerous not only for the United States, but for Europe and the rest of the world as well."
Steinmeier, a Social Democrat, has blasted Trump for months for his "America First" speech, while praising Democrat Hillary Clinton as an experienced foreign policy expert.
Trump on Tuesday secured the party's 2016 nomination for the White House after vanquishing 16 party rivals, warring with much of his party's establishment and already provoking controversy at the convention.
Steinmeier said the current crisis-charged world situation tempted people to look for easy answers. However the challenge was to find solutions that worked and did not rely on simple slogans or the erection of new barriers, he said.
Trump's comments during the campaign - including calls to ban Muslims from the United States - have raised alarm in allied countries unnerved by the phrase "America First", used in the 1930s by isolationists who sought to keep the United States out of World War Two.