"The first threat, an external one, is related to the new geopolitical situation in the world and around Europe", the letter said.
"The change in Washington puts the European Union in a hard situation; with the new administration seeming to put into question the last 70 years of American foreign policy", Tusk told the 27 heads of state and government.
Tusk's letter came a few days before European Union leaders are due to meet in Malta to discuss the future of the European Union as a 27-member state bloc.
European diplomats said senior national officials and diplomats discussed a possible EU response to Trump at a meeting in Brussels on Monday.
President Donald Trump was named Tuesday as an external threat to Europe by European Council President Donald Tusk, in which the USA leader joined the likes of Russia, China and radical Islam. The decision to leave was the biggest setback for the European Union in decades, and Trump didn't endear himself with many European Union leaders by saying that Brexit "will be a tremendous asset and not a tremendous liability".
Trump has repeatedly questioned the value of American participation in North Atlantic Treaty Organisation, although he has simultaneously made assurances that the United States remains committed to the military alliance.
Most EU officials have been disconcerted by the remarks of the U.S. president, who has never served in a government or diplomatic post.
Meanwhile, in an interview with the Financial Times, published today, Trump's top trade adviser Peter Navarro called the euro a "grossly undervalued" currency, adding it was being used by Germany to "exploit" the United States and give the country an unfair edge over its trading partners.
Many European leaders were categorical in their condemnation of the suspension of immigration from seven Muslim-majority countries and of refugee resettlement.
Writing a letter to all the 27 European Union leaders Tusk said the new presidency is contributing to unpredictable outlook for the bloc in the first weeks following oath taking.
"I have made my mind up that in the coming years we are on our own, which may be a good thing. We should remind our American friends of their own motto: United we stand, divided we fall".
Tusk later met with the heads of E.U. Baltic member states Estonia, Latvia and Lithuania where he again emphasized the need for Europeans to come together against their foes, including Trump.