"It is a historic day, the most important day since independence" from Moscow in 1991, Poroshenko said.
Ukraine will use "the opportunity to modernise," he said, stressing the importance of peace and security for its future and that of the region.
The accord was a "demonstration of EU solidarity," he added.
EU leaders will sign association accords with Ukraine plus Georgia and Moldova at 0700 GMT, marking a decisive turn to the West for the three former Soviet republics.
The last-minute ditching of the association accord by Ukraine's pro-Russian president Viktor Yanukovych in November sparked demonstrations that eventually led to his ouster by pro-EU forces in February.
That in turn led to an angry Russia annexing Ukraine's Crimea peninsula, plunging the West's relations with the Kremlin to their lowest level since the end of the Cold War.