Mr Tusk used Twitter on Thursday to back the Democratic presidential nominee, Mr Biden, and mock US president Donald Trump.
That endorsement came despite Mr Tusk saying that he “always believed in the Republican ideals and greatness of America”.
Mr Tusk, who served as European Council president between 2014 and 2019, said on Thursday that he “got to know @realDonaldTrump really well” whilst president.
“These are the reasons why I pray for Joe Biden’s success,” wrote Mr Tusk, who added that US president Ronald Reagan had been his “hero”.
Thursday’s statement comes some months after Mr Tusk, who now leads the centre-right European People’s Party, last criticised the US president in public.
In a speech last September, Mr Tusk told the UN General Assembly that “To protect the truth, it is not enough to accuse others of promoting fake news. Frankly speaking, it would be enough to simply stop lying.”
That statement, whilst not naming Mr Trump, alluded to the US president’s unsubstantiated complaints about American media.
Mr Tusk also appeared to condemn America’s turn towards isolationism, and said: “The patriotism of the 21st century must also have a global dimension, if it is not to become, as has many times been the case, a common national egoism.”
Some weeks later, Mr Tusk tweeted a picture in which he held two fingers against president Trump’s back at a NATO summit in London.
Mr Tusk commented that “Despite seasonal turbulences our transatlantic friendship must last.”