He praised Poland both on its achievement of the investment threshold as well as on its Wednesday decision to go ahead with the purchase of Patriot missile defence systems from the US.
In his first public comments since North Korea tested an intercontinental ballistic missile for the first time this week, Trump declined to offer specifics about what a US response might entail, though he called it a "threat" and said the USA would "confront it very strongly".
On North Korea, U.S. officials confirmed that the country launched its first successful intercontinental ballistic missile test this week, just days before Trump was scheduled to travel to Europe for the G-20 summit in Germany.
In the speech, President Trump renewed his call for nations to unite in the fight against terrorism, "to strip them of their territory, their funding, their networks, and any form of ideological support".
"Trump is going to the G20 summit as a politician who has isolated himself from the Paris Agreement", said Pawel Szypulski of Greenpeace Poland.
"I think it was Russian Federation, I think it could have been other countries", Trump said.
Trump first noted that Poland's "victory over communism" was supported by a "strong alliance in the West that defied tyranny".
Trump had notably omitted such a mention during a speech in late May at NATO's new headquarters in Brussels.
"To those who would criticize out tough stance, I would point out that the United States has demonstrated not merely with words but with its actions that we stand firmly behind Article 5, the mutual defense commitment. Bad thing", Trump added.
Many Poles welcome strengthened bonds with the United States, says Marcin Zaborowski of Visegrad Insight, a Warsaw-based journal of politics and culture focused on central Europe.
Asked if he can definitively confirm Russian interference in the 2016 election, President Trump was somewhat vague. "To meet new forms of aggression, including propaganda, financial crimes and cyber warfare, we must adapt our alliance to compete effectively, in new ways and on new battlefields".