He cited his relationship with Polish President Karol Nawrocki, whom he endorsed in last year's election.
"Based on the successful election of the now President of Poland, Karol Nawrocki, who I was proud to endorse, and our relationship with him, I am pleased to announce that the United States will be sending an additional 5,000 troops to Poland," Trump wrote on Truth Social.
Nawrocki thanked Trump in a post on X, saying: "I stand and will continue to stand guard over the Polish-American alliance—an important pillar of security for every Polish home and for all of Europe."
He added that "good alliances are those based on cooperation, mutual respect and commitment to our shared security."
Nawrocki also thanked Trump "for his friendship toward Poland and for his decisions, the practical implications of which we can see very clearly today."
Polish Deputy Prime Minister and Defence Minister Władysław Kosiniak-Kamysz said Trump’s decision "confirms that Polish-American relations are very strong and that Poland is a model and steadfast ally."
"It is good that we are fighting together for issues fundamental to the security of our country,” Kosiniak-Kamysz wrote on X. "This is a duty that translates into effectiveness."
Trump's surprise announcement came a week after the Pentagon halted a planned deployment of 4,000 troops to Poland.
The Pentagon had earlier announced plans to withdraw 5,000 troops from Germany and indicated that further reductions of US forces in Europe could follow.
Polish Deputy Defence Minister Cezary Tomczyk said on Wednesday, after talks with White House officials in Washington, that the United States intended to maintain a strong military presence in Poland and consult Warsaw on any future troop deployments in Europe.
"Donald Trump has just announced that an additional 5,000 US troops will be deployed to Poland," Tomczyk said in a video message on Thursday. "In situations like this, one thing is certain: all hands need to be on deck. Pressure produces results."