The meeting brought together the leaders of Poland, France, Germany, Italy and Britain, a grouping known as the European Group of Five (E5), ahead of next month's NATO summit in Ankara.
NATO Secretary-General Mark Rutte joined the discussions remotely.
"Our countries will safeguard European and transatlantic unity," Tusk told a joint news conference after the meeting at the German Chancellery.
"The future of Europe, the Western world and Ukraine will depend on how well we preserve European and transatlantic unity," he said, adding that the message should be clear ahead of the NATO summit in Turkey.
Tusk said Poland would work to ensure that countries on NATO's eastern flank, which face the most direct security threats from Russia, are represented in all major discussions on European security.
"Without them, it will be difficult to achieve anything in this civilizational confrontation with the aggressive neighbours in the east," he said.
The Polish leader also pointed to practical measures taken by Warsaw, including defence spending "approaching 7 percent" of gross domestic product.
"Those who take their commitments seriously should make a greater effort to strengthen Europe's and their own defence capabilities," Tusk said.
German Chancellor Friedrich Merz, who hosted the meeting, said the E5 leaders had agreed on five key messages they would promote ahead of the NATO summit.
The first was support for NATO as the cornerstone of transatlantic security, followed by efforts to strengthen the alliance's European pillar through increased defence spending and military capabilities, Merz said.
He also stressed the need for closer coordination among European countries and continued cooperation with the United States.
"We want to act as Europeans, but also in transatlantic cooperation with the Americans," Merz said, according to Poland's PAP news agency.
A fourth priority was continued support for Ukraine, including long-term financial assistance, he added.
"We must show Russia that Ukraine will remain strong because European support will not weaken," Merz said.
The German chancellor also welcomed a recent framework agreement between the United States and Iran and voiced support for continued diplomatic engagement between Washington and Tehran.
Merz said Europe and the United States were now more closely aligned than they had been in years and argued that conditions were emerging for peace talks between Ukraine and Russia.
Italian Prime Minister Giorgia Meloni said Europe must continue supporting Ukraine until a "lasting and just peace" is achieved.
"The world is currently at a historic moment in which a new global security architecture is taking shape," she said.
British Prime Minister Keir Starmer argued that growing dissatisfaction with Russian President Vladimir Putin inside Russia created an opportunity to increase pressure on Moscow, tighten sanctions and increase support for Ukraine.
Starmer added that Ukrainian forces had recently succeeded in halting Russian advances on the battlefield.
French President Emmanuel Macron said the United States and Europe were moving closer together on both Ukraine and Iran.
He said Washington increasingly recognised the importance of preserving Ukraine's territorial integrity and was prepared to consider additional sanctions against Russia.
"Our discussions in Ankara will address these issues," Macron said.
The E5 group, formed in 2024, brings together Europe's largest defence spenders—Poland, France, Germany, Italy and Britain—to coordinate support for Ukraine and broader European security policy.
The Berlin meeting came weeks after a smaller gathering of the leaders of Britain, France and Germany in London, where they met with Ukrainian President Volodymyr Zelensky and outlined conditions for ending the war in Ukraine.
Tusk at the time criticised Poland's exclusion from those talks.
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