The senior politician said he had been asked by British friends if there was a way to stop or reverse Britain’s exit from the EU.
Mr Tusk said: “I told them that in fact the EU was built on dreams that seemed impossible to achieve.
"So who knows? You may say I am a dreamer but I'm not the only one," he added, quoting John Lennon's "Imagine".
His remarks came hours before EU leaders meet in Brussels to discuss terrorism and security.
But Brexit is also likely to dominate, with Theresa May set to update her European colleagues on the future for EU citizens living in the UK.
The two-day summit, Ms May’s first outing with EU counterparts since her lacklustre performance at the general election, follows four days after Brexit talks officially began.
EU leaders have gained renewed confidence after pro-Europe Emmanuel Macron's victory in the French poll last month.
Speaking at a news conference in Brussels, Mr Tusk said the EU is "slowly turning the corner".
"We are witnessing the return of the EU rather as a solution, not a problem."
He continued: "The Brexit negotiations started three days ago. It is a most difficult process, for which the EU is well prepared.
"You can hear different predictions coming from different people about the possible outcome of these negotiations - hard Brexit, soft Brexit or no deal.
"Some of my British friends have even asked me whether Brexit could be reversed and whether I could imagine an outcome where the UK stays part of the EU.
"I told them that, in fact, the European Union was built on dreams that seemed impossible to achieve. So, who knows. You may say I'm a dreamer, but I'm not the only one."
Mr Tusk's suggestion that Brexit could be stopped, reversed or watered down, followed similar remarks made by the newly-elected French President, who told Ms May at a joint news conference in Paris last week that “the door remains open” for Britain to change its mind.
“Of course the door remains open, always open until the Brexit negotiations come to an end,” he said, when asked if Britain could yet stay in the bloc.
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies