He said that "worrying declarations from President Trump are among the challenges faced by the EU," and that the change in Washington was part of an external threat that also included an assertive China, an aggressive Russia and radical Islam.
His comments come in a letter to 27 European leaders ahead of an EU summit in Malta, current holder of the EU Council presidency, this Friday.
In the letter, Tusk also said he believed most EU leaders agreed with him.
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Several statements from Washington have prompted alarm in Europe's capitals.
Tusk said the new US administration placed the EU in a "difficult situation" as it appeared to "put into question the last 70 years of American foreign policy".
"We cannot surrender to those who want to weaken or invalidate the transatlantic bond, without which global order and peace cannot survive. We should remind our American friends of their own motto: United we stand, divided we fall."
On Tuesday, members of Parliament's development and foreign affairs committees issued a joint resolution saying that the US administration's ban on citizens from seven Muslim-majority states entering the country could "seriously undermine current global efforts towards a fair international sharing of responsibilities for refugees."
The resolution said the US decision "fuels anti-immigration and xenophobic discourses", and could "seriously undermine current global efforts towards a fair international sharing of responsibilities for refugees."
The signatories called on the EU to "speak with one voice to defend the international protection system and the legal security of all affected populations."
Further condemnation came from Parliament's anti-racism and diversity intergroup (ARDI), which issued a statement saying the order "breaches both federal and constitutional law."
The statement read, "The United Nations 1951 Refugee Convention, which is incorporated into United States law, prohibits discrimination against refugees on the basis of religion."
Belgian Alde MEP Gérard Deprez, the co-president of ARDI, said, "The fight against terrorism is an absolute necessity but it does not justify widespread suspicion against persons according to their country of origin, belief or religion."
He added, "We, as Europeans, cannot close our eyes to what is happening in the US. I therefore express my deep concern about the executive order and I hope that European leaders and the Commission will not remain spectators, that we will react, in particular to defend EU citizens who will no longer be able to travel to the US because of their dual nationality.
"It is also our duty to do everything we can to ensure respect for the Geneva convention with regard to the rights of refugees."