He laid out the agenda for discussions at the important meeting, with migration topping the list.
Writing on the issue of transatlantic relations, Tusk said the EU must be prepared for "worst-case scenarios" as US President Donald Trump's policies have been increasingly at loggerheads with the bloc's values.
"It is my belief that, while hoping for the best, we must be ready to prepare our union for worst-case scenarios," Tusk wrote. "Despite our tireless efforts to keep the unity of the West, transatlantic relations are under immense pressure due to the policies of President Trump."
Trump has decided to withdraw his country from the Paris climate deal and the Iran nuclear deal, despite repeated pleas by the EU to stick with them.
One EU official, who spoke on the condition of anonymity, told DPA news agency that such "negative" decisions were starting to "look like a pattern" where the US has "no friends, no enemies" and where preserving the international rules-based structure was not a focus.
NATO Secretary General Jens Stoltenberg will share his views on EU-NATO cooperation at the meeting.
Read more: Donald Tusk warns EU leaders to ″prepare for the worst″ in EU-US relations | News | DW | 27.06.2018