Amid speculation that Trump would refuse to meet the Ukrainian leader, Poroshenko had what the White House described as a "drop-in" with the US president.
"We've had some very, very good discussions," Trump said as the pair sat side-by-side in the Oval Office.
"I think a lot of progress has been made," Trump added, saying it was a "great honor" to host Poroshenko.
Trump did not touch on the thorny details of the relationship.
For three years, ties between Washington and Kiev have been dominated by efforts to contain a destabilizing Russian-backed rebellion in eastern Ukraine.
The crisis has left 10,000 dead and heightened tensions between the West and the Kremlin, which considers Ukraine inside its "sphere of influence."
Ukrainian officials have long been frustrated by Washington's refusal to provide lethal weapons and its insistence on linking financial help to deep economic reforms.
But that concern has intensified since the election of Trump, who has often appeared reluctant to put pressure on Russian President Vladimir Putin to solve the crisis.
Last week, Secretary of State Rex Tillerson raised eyebrows by suggesting that the United States should not be "handcuffed" by an international plan to end the crisis.
Trump did not speak publicly about either of those two substantive issues, but described Ukraine as "a place that we've all been very much involved in and we've been seeing it and everybody has been reading about it."
The sit-down was not announced in advance by the White House -- as is customary -- and when it was described, it was billed as something less than a full bilateral.
In the world of diplomacy, where symbolic actions can have real consequences on the ground, no meeting at all would have been seen as a snub.
"We're really fighting for freedom and democracy," Poroshenko told Trump, describing the United States as a "partner."
"It's a great honor, a great pleasure to be together with you, Mr President."
Shortly before the meeting, the US Treasury announced a fresh round of economic sanctions on Russia and Ukrainian separatists.
The 38 individuals and organizations named included two Russian government officials and 11 others who operate in the annexed Ukrainian region of Crimea.
Among the individuals sanctioned was Petr Jarosh, head of the Russian Federal Migration Service in Crimea and Alexander Babakov, a Russian official liaising with Russian expatriates.
The Treasury also barred Americans from offering new loans of greater than 90 days to 20 subsidiaries of the Russian energy firm Transneft, which already was subject to US sanctions.
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