The European Council President kicked off the latest European summit, held in the Bulgarian capital Sofia, telling reporters Mr Trump had become a far from useful ally who acts with “capricious assertiveness”.
Washington and Brussels have been at loggerheads over a number of issues since Mr Trump was inaugurated in January 2017, most recently a trade war of aluminium and steel tariffs and the Iran nuclear deal.
Speaking to reporters in Sofia, Mr Tusk said the US President had “made us realise that if you need a helping hand, you all find one at the end of your arm”.
The Council President urged Europe to ditch its American partnership and go alone as the EU faces up to global challenges because of Mr Trump’s unreliability.
Mr Tusk said: “Besides traditional political challenges, such as the rise of China or the aggressive stance of Russia, we are witnessing today a new phenomenon: the capricious assertiveness of the American administration.
“Looking at the latest decisions of President Trump, someone could even think: with friends like that, who needs enemies?
“But frankly speaking, Europe should be grateful for President Trump because thanks to him we have got rid of all illusions.
“He has made us realise that if you need a helping hand, you will find one at the end of your arm.”
He added: “Europe must do everything in its power to protect, in spit of today’s mood, the transatlantic bond, but at the same time we must be prepared for those scenarios where we have to act on our own.
“We have enough potential to rise to the challenge, but what we need is more political unity and determination.”
European leaders will gather in Sofia to mainly discuss the EU’s commitment towards enlargement talks focused around Western Balkan regions, but the US President’s actions have seemingly hijacked the agenda.
The bloc’s leaders will hope to present a united front against Mr Trump in response to his decision on the Iran nuclear deal and trade tariffs.
Theresa May will travel to the Bulgarian capital where she will join a working dinner with her fellow European leaders, including European Commission President Jean-Claude Juncker.
There is likely to be little or no Brexit discussions as the British Prime Minster alongside Angela Merkel and Emmanuel Macron will “present their assessment of the situation” surrounding the Iran nuclear deal, according to Mr Tusk.