Ukraine's election commission chief said that the tycoon won the election in the first round by collecting more than half the votes in a poll that Washington hopes will bring stability to the eastern European country.
"We will support their efforts to determine their own future in a more united, secure, independent, and prosperous Ukraine," Kerry said in a statement before Poroshenko's victory was announced.
Yesterday's vote was seen as the most important in Ukraine's post-Soviet history as it fights to stay united after months of turmoil and avert economic collapse.
"The successful conduct of these elections reaffirms Ukraine's commitment to the democratic process. The United States will continue to work with the people of Ukraine and the newly elected President to build on this victory for democracy," said Kerry.
He added: "The United States respects Ukraine's sovereignty and territorial integrity, condemns and rejects Russia's occupation and attempted annexation of Crimea, and remains committed to working with Ukraine and other partners to find a peaceful resolution to the conflict."