Poland in return freed Alexander Butyagin, a Russian archaeologist whom it had been preparing to extradite to Ukraine.
Tusk said five prisoners were freed for five Belarusian or Russian citizens, including Polish priest Grzegorz Gawel and an unnamed Belarusian who Tusk said co-operated with Polish special services.
Not all of the names of the released individuals were immediately announced.
It was also not immediately clear who, apart from Butyagin, was handed over in exchange.
A Belarusian of Polish origin, Poczobut was arrested in March 2021 and sentenced in 2023 to eight years in prison on charges of inciting ethnic hostility and undermining Belarusian security.
Poland says the charges were unjust and politically motivated, and had long sought his release.
"Andrzej Poczobut is free! Welcome to your Polish home, my friend," Tusk wrote on social media platform X.
Tusk described Poczobut as "unyielding", quoting their first conversation after his release: "'Will I be able to go back there (to Belarus)?' - those were his first words. 'Only you decide. You are now a free man,' I replied."
Poczobut received the European Union's 2025 Sakharov Prize for Freedom of Thought, a human rights award.
Gawel, a Carmelite priest from Krakow, was detained by Belarusian authorities last year on charges of espionage.
Poland has become a refuge for opponents of Belarusian President Alexander Lukashenko and a staunch supporter of Ukraine since Russia, Minsk's main ally, launched its full-scale invasion in 2022.
Lukashenko has released hundreds of prisoners in the past two years, a process that has accelerated since Donald Trump returned to the White House and sent a special envoy, John Coale, to negotiate with him.
The US in return has begun removing sanctions against Belarus.
Coale told Reuters on Tuesday that he expects to secure the release of more prisoners from Belarus in the next month, adding that the lifting of further sanctions on Minsk was always a possibility if this happens.
Human rights groups say more than 830 political prisoners still remain in Belarusian jails.
Russia's FSB security service, quoted by state news agency TASS, said two Russian citizens were coming home under the swap.
One was Butyagin, arrested in Poland in December.
He had been due to be handed over to Ukraine, which accused him of unauthorised excavations and plundering artefacts in Crimea.
Russia had expressed outrage over his arrest and demanded his release.
The FSB said the other Russian who was released was the wife of a Russian soldier serving with Russian forces in Transdniestria, a breakaway region of Moldova.
It said the two Russians were exchanged for two Moldovan spies who arrived in Russia last year and were detained by Russian security agencies.
Moldovan President Maia Sandu wrote on X: "Today, we are bringing two Moldovan citizens home from Russian captivity. This would not have been possible without @realDonaldTrump and the US administration, and our partners in Poland and Romania. We are deeply grateful. They're coming home!"
