Speaking ahead of a Brussels summit where Prime Minister Theresa May briefed leaders on her Brexit plans, Tusk suggested that if Britain wanted to reverse the process, they could work it out.
"Some of my British friends have asked me whether Brexit could be reversed, and whether I could imagine an outcome where the UK stays part of the European Union," Tusk told reporters.
"I told them that in fact the European Union was built on dreams that seemed impossible to achieve, so who knows?" the former Polish premier added.
"You may say I am a dreamer, but I am not the only one."
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Tusk's last line was a quote from "Imagine", the iconic 1971 ode to world peace by Lennon, who was gunned down by a deranged fan in 1980.
Tusk said later that his upbringing in communist Poland had taught him that "miracles are possible" in politics, adding however that the EU had to take a "realist" stance and move on with Brexit negotiations.
Britain stunned the EU when it voted to end its four-decade membership of the 28-nation bloc in a referendum a year ago on Friday.
But May's disastrous showing in elections on June 8 in which she lost her parliamentary majority has sparked speculation that her Brexit plans may be watered down, or even reversed.
The EU chief's comments provoked mixed reaction among leaders of the bloc, as well as debate about whether he was being serious.
But Belgian Prime Minister Charles Michel -- who has strongly argued for EU unity on Brexit -- said the issue was closed.
"I am not a dreamer and I am not the only one," Michel told reporters, adding that he was only a "dreamer in my private life."
"I understand it was British humour by Donald Tusk," he added.
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'Let it be'
European Commission chief Jean-Claude Juncker also said the EU should move on.
"In Europe I never have illusions because I don't want to lose them," he said. "Let it be."
Calling Brexit "a pity", Lithuania's outspoken President Dalia Grybauskaite said: "We need to think about the future and the sooner we settle the future the better for all of us."
But Netherlands Prime Minister Mark Rutte said he shared Tusk's fantasy of Britain somehow staying in the bloc.
"We all have that dream. I hate Brexit from every angle," he told reporters.
"My dream would be... we would come to a sort of intermediate end state for the coming years in which the United Kingdom would stay connected to the single market."
French President Emmanuel Macron and German Finance Minister Wolfgang Schaeuble both said last week that the "door was open" for Britain to remain in the EU.
May is set for a hard day's night at the summit as she unveils an offer to resolve EU concerns about the rights of three million European expats living in Britain after Brexit.
Tusk meanwhile insisted the remaining 27 EU members had a renewed sense of optimism about the bloc's future after years of crisis and mounting anti-EU sentiment culminating in the Brexit vote.
Despite it being his 80th summit as premier or EU head, "never before have I had such a strong belief that things are going in a better direction," he said.
"Our optimism should still be extremely cautious but we have good reason to talk about it."
Tusk later confirmed his position on one of the music world's most enduring debates -- who is the better Beatle out of Lennon and his songwriting partner Paul McCartney.
"Of course I prefer John Lennon," Tusk told reporters.
