"1000 days of Russian aggression, 1000 days of Ukrainian , 1000 days of Polish aid and solidarity," Tusk wrote on the X social media platform on Tuesday.
He added: "By supporting Ukraine, we’re moving the war away from our Polish and European borders. Let us remember that every single day."
Ukrainian President Volodymyr Zelensky shared a video on X, paying tribute to his nation on this sombre milestone.
Zelensky on Tuesday addressed European lawmakers remotely in a special plenary session marking 1,000 days of Russia's war against Ukraine.
Russia invaded Ukraine by land, air and sea on February 24, 2022, starting the largest military conflict in Europe since World War II.