May 27, 2017 0911 GMT
In an interview with Bild am Sonntag newspaper, Juncker raised concerns that the Trump administration could impose punitive tariffs on European products, causing the EU to do the same with US products.
“A trade war would neither be in Europe’s nor the USA's interests,” he told the German publication.
The EU Commission president further criticized Trump for welcoming Brexit and urging other EU members to follow Britain's example in leaving the bloc.
He added the policy shows the Republican president has “an uncanny approach” toward the relations with the European Union.
Europe “must take the protectionist rhetoric of the Trump administration seriously and be prepared,” Juncker said.
Trump has raised serious concerns about the future of free trade and has promised to withdraw from or renegotiate trade agreements such the Trans-Pacific Partnership (TPP) and the North American Free Trade Association (NAFTA).
In his interview, Juncker said other EU members would not consider leaving the bloc because Brexit will make them realize it is not a good option.
"Britain's example will make everyone realize that it's not worth leaving," he told the newspaper.
"On the contrary, the remaining member states will fall in love with each other again and renew their vows with the European Union," he added.
During a gathering in Rome next week, member nations plan to declare "Europe is our common future" to mark 60 years of the EU.
Juncker said the UK would need to get used to the fact that it is no longer seen as a member.
"Half memberships and cherry-picking aren't possible. In Europe you eat what's on the table or you don't sit at the table," he added.
British Prime Minister Theresa May, who is expected to trigger the Brexit process late in March, intends to take Britain out of the European single market, potentially damaging London's role as a financial hub.