Tusk directed his message at Trump, saying "appreciate your allies, after all you don't have that many."
His blunt remarks came as he signed a joint statement on cooperation between the European Union and the North Atlantic Treaty Organization.
NATO has been struggling to put on a display of unity for a two-day summit at its new headquarters, amid deep divisions among member countries over Trump's policies on trade and his decision to withdraw from the Iran nuclear deal and global climate agreement.
"America does not have and will not have a better ally than Europe today. Europeans spend on defense many times more than Russia and as much as China and I think you can have no doubt, Mr. President, that this is an investment in common American and European defense and security which cannot be said with confidence about Russian or Chinese spending," Tusk said.
Trump has routinely criticized his NATO allies for failing to spend enough on their defense budgets and he is likely to repeat his demands for more money in Brussels on Wednesday.
Tusk also urge Trump to remember who are friends when he meets Russian President Vladimir Putin next week.
Trump said on Tuesday he could not say if Putin is a friend or foe, and that a meeting with the Russian president could be the easiest part of a tour of Europe that includes a NATO meeting and a visit to Britain.
Speaking to reporters at the White House before leaving on the week-long trip, Trump repeated his criticism of NATO allies for not spending enough on their defense and pointed to the political tensions in Britain over the government's Brexit plans.
"So I have NATO, I have the UK which is in somewhat turmoil, and I have Putin. Frankly Putin may be the easiest of them all. Who would think?" Trump said.
"We'll see how that goes," he said of the summit with Putin to be held in Helsinki on Monday.
Asked if Putin is a friend or foe, Trump said: "I really can't say right now. As far as I'm concerned, a competitor."
AP and Reuters contributed to this story.