President Andrzej Duda swore in the new prime minister and his Cabinet during a high-profile ceremony in the presidential palace, Polish state news agency PAP reported.
The swearing-in ceremony came after parliament gave its vote of confidence to Tusk and his team of ministers on Tuesday night.
The new government was formed after a bloc of parties led by Tusk's liberal Civic Coalition (KO) stormed to power with a strong showing in Poland's October 15 parliamentary elections.
At Wednesday's ceremony, the president congratulated the new governing coalition on its "determination" to form a joint Cabinet and on its decision to "take responsibility for Polish affairs."
Duda said he was "open to working together" with the new Cabinet, on issues such as climate, national security and research and development, private broadcaster Radio Zet reported.
The president invited Tusk and his ministers to a meeting of the National Security Council next Wednesday "to discuss Poland's security situation."
Duda also declared he would not block legislation that "benefits Poland."
The new prime minister, Donald Tusk, thanked fellow citizens "regardless of who they voted for on October 15," and said he was happy with the president's "willingness to cooperate" with his Cabinet.
The swearing-in ceremony ended eight years of rule by the conservative Law and Justice (PiS) party.
In a major policy speech on Tuesday, Tusk vowed to raise public-sector pay and unblock EUR 59.8 billion in grants and loans from the European Union, frozen due to Warsaw's dispute with Brussels over democratic standards.
The incoming prime minister also pledged that, under his government, Poland would be "a strong link in NATO," a stable ally for America and regain its "rightful position" in the EU.
Tusk, a former top EU official, also told deputies that his government would work to ensure that Poland's EU partners "unequivocally support Ukraine's defence against Russian aggression."
Tusk previously served as Poland's prime minister from 2007 to 2014 before becoming president of the European Council, a role he held until 2019.
(pm/gs)
Source: PAP, prezydent.pl, Radio Zet
Click on the audio player above for a report by Radio Poland's Michał Owczarek.