But the insurgency, which has already cost at least 150 lives, thwarted polling in much of the east and rebels have defiantly refused to recognize the result.
Russia, which has been threatened with a new round of Western sanctions if it meddled further in Ukraine after its seizure of Crimea in March, said however it was willing to work with the new leaders.
"We are ready for pragmatic dialogue, on an equal footing, based on respect for all agreements, in particular in the commercial, economic and gas spheres," Foreign Minister Sergei Lavrov said in Russia's first reaction.
"As the president (Vladimir Putin) has said, we respect the result of the choice of the Ukrainian people."
Rebels have rejected dialogue with Poroshenko, with Donetsk separatist leader Denis Pushilin saying talks were only possible if Russia mediated and calling for a prisoner swap and a withdrawal of Kiev's troops.
But despite their efforts to thwart the vote, observers with the Organisation for Cooperation and Security in Europe said the election "largely upheld democratic commitments" and provided the new leader with legitimacy despite the problems in the east.