"I sincerely congratulate Donald Trump on his election as president of the United States," Poroshenko told US Ambassador to Kiev Marie Yovanovitch, also inviting Trump for a visit, according to a statement from the presidency.
He said he hoped for "continuing support of the US... in Ukraine's fight against Russian aggression, for freedom, independence and restoration of sovereignty and territorial integrity," referring to the pro-Russian separatist regions in the east.
Nearly 10,000 people have been killed since the Ukraine conflict began in 2014, pitting pro-Russian rebels against government troops.
Some in Ukraine have expressed worry that Trump, who praised Russian President Vladimir Putin during the campaign, would sacrifice Ukraine to improve relations with Moscow.
Trump's campaign reportedly snubbed Poroshenko when the Ukrainian leader reached out to meet on the sidelines of the UN General Assembly this year, though he was able to meet with Clinton.
Trump has given muddled statements on Ukraine, calling it "a mess" and saying the people of Crimea have chosen to be with Russia, in an apparent reference to the controversial referendum that Moscow used to justify its annexation of the peninsula in 2014.
Russia denies either sending troops or weapons across its border to fuel the conflict, despite eyewitness testimony to the contrary. But it openly back the separatists' cause at international venues such as the UN Security Council.