Ukrainian President Volodymyr Zelensky signed the agreement, along with European Commission President Ursula von der Leyen and European Council President Charles Michel.
Under the agreement, Ukraine secured guarantees of political, financial, economic, humanitarian, military and diplomatic support from the 27-nation bloc, news outlets reported.
Assistance from Western allies will be available to Ukrainian citizens for as long and as intensively as needed, officials said.
The signing ceremony took place in Brussels just before the start of a summit of EU leaders, Polish state news agency PAP reported.
On the sidelines of the summit, Ukraine also concluded bilateral agreements with EU member states Lithuania and Estonia, with Zelensky signing the documents together with Lithuanian President Gitanas Nausėda and Estonian Prime Minister Kaja Kallas.
Lithuania's Nausėda said in an X post that his country's security agreement with Ukraine "confirms our unwavering support for Ukraine's battle for freedom."
Nausėda added: "Lithuania has stood with Ukraine since the beginning of Russia's brutal war and will continue to stand together until victory."
Polish PM, Ukraine's Zelensky to meet in Warsaw
Poland plans to offer similar guarantees to Ukraine, Prime Minister Donald Tusk confirmed.
He said during the summit that preparations were nearing completion for Poland to sign a bilateral security agreement with Ukraine.
Tusk announced on X that he and Zelensky would meet in Warsaw before NATO's Washington summit.
"The European Union has understood what Poles have known since the beginning of this war: that defending Ukraine is defending Europe," Tusk said in a post.
So far, a total of 17 countries have signed security agreements with Ukraine, including some EU member states.
Following decisions made at last year's NATO summit in Vilnius, the United States and Japan have also concluded agreements guaranteeing support for Ukraine.
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Source: IAR, PAP