"Poland will for sure be a country that encourages common sense and caution," Tusk told reporters in Brussels on Monday morning.
"But we also have to say clearly that any plans to introduce customs duties and wage trade wars are a complete mistake," he added.
He was speaking as European Union leaders gathered in the Belgian capital for an informal summit to discuss transatlantic relations and ways to jointly finance defence projects.
"I hope that such a firm, clear, friendly, but tough European position will get across to the American administration," Tusk said, two weeks after Trump returned to the US presidency..
EU leaders are meeting on Monday, bracing for the possibility that the 27-nation bloc too will face US trade levies, the Reuters news agency reported.
Before leaving for Brussels, Tusk told reporters in Warsaw that the EU "must be unquestionably united."
He added: "It is worth doing everything possible to ensure that in the face of a Russian threat or Chinese expansion, we do not fight between allies."
Monday's summit in Brussels is scheduled to conclude with a press conference in the evening featuring Tusk in his role as the leader of the EU’s rotating presidency, alongside European Council President António Costa and European Commission chief Ursula von der Leyen, Polish state news agency PAP reported.
Trump reiterated on Sunday that he would impose tariffs on the European Union, according to media reports.
Speaking to journalists at Joint Base Andrews in Maryland, Trump said that there was no timeline yet but that it could happen "pretty soon," public broadcaster Polish Radio's US correspondent, Marek Wałkuski, reported.
"They don't take our cars, they don't take our farm products, they take almost nothing and we take everything from them: millions of cars, tremendous amounts of food and farm products," Trump said, as cited by British broadcaster BBC.
When asked if there was a timeline for announcing tariffs on the EU, Trump said: "I wouldn't say there's a timeline, but it's going to be pretty soon," the BBC reported.
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Source: IAR, PAP, Reuters