Trump spent much of Thursday hosting Polish president Andrzej Duda and his wife Agata Kornhauser-Duda at the White House to talk defence.
The US will send 1,000 more troops to Poland as part of a growing security and economic partnership between the two countries, the president subsequently announced.
But Trump and Duda differed over Russia's intentions toward the US ally.
At a time of ongoing worries about Russian military activity, Duda said he wanted Russia to be a friend of Poland even as he recounted his country's long history of conflict with Moscow.
"We would like Russia to be our friend, but unfortunately, Russia again is showing its very unkind, unpleasant imperial face," Duda said, noting its attacks on Georgia in 2008 and Ukraine in 2014.
Trump, who has a history of appearing to defer to Russia, seemed to downplay Duda's concerns.
"I hope that Poland is going to have a great relationship with Russia. I think it's possible. I really do," Trump said.
"I think because of what you've done, and the strength, and maybe we help also, because of what we're doing and doing for Poland.
"But I hope Poland is going to have a great relationship with Russia. I hope we're going to have a great relationship with Russia, and by the way, China and many other countries," he said.
Trump said he likely will make his second visit to Poland as president in September, the 80th anniversary of the invasion by Nazi Germany in 1939 that set off the Second World War.
He said he was thinking about allowing Poland to participate in a State Department programme that allows its citizens to visit the US for tourism or business without obtaining a visa, but that more progress was needed before a final decision.
Polish leaders had hoped to land a permanent US base in their country that they said could be called Fort Trump, but the agreement announced on Wednesday fell short of that. It includes just the addition of about 1,000 troops and a squadron of Reaper drones for intelligence purposes.
Trump said the Polish government will pay for the infrastructure to support the troops. The service members will be added to an existing force of about 4,500 US troops that rotate in and out of Poland.
Trump said he probably would shift some US service members from Germany, where tens of thousands have been based for a "long, long time," or from elsewhere in Europe.
He offered no timetable for when the additional military personnel would begin arriving in Poland.
Trump also hailed Poland's decision to buy more than 30 F-35 joint strike fighter jets from the US.
Duda also invoked Trump's "Make America Great Again" campaign slogan as he thanked the president for his interest in Poland.In the Oval Office, Trump said he had no concerns about backsliding on democracy in Poland.
Duda denied there were problems, saying "everything is excellent". Critics have accused the Polish government of taking steps to undermine the judiciary and the news media.