Pellegrini, a nationalist-left government candidate, defeated pro-Western opposition candidate Ivan Korčok by a comfortable margin in the second round of the Slovak presidential vote on Saturday, news agencies reported.
Polish President Andrzej Duda said in an X post: "Our southern neighbours, the Slovaks, have elected a new president. I congratulate President-Elect Peter Pellegrini on his successful campaign and the trust his fellow citizens have placed in him."
Duda added: "I look forward to our continued tradition of excellent neighbourly cooperation."
Pellegrini's win cements the grip of pro-Russian Prime Minister Robert Fico over the country, the Reuters news agency reported.
Pellegrini, 48, said at his campaign headquarters that he would be "a president who will support the government in its efforts for improving people's lives."
He also vowed efforts to ensure that his country "remains a strong member of the European Union and NATO," according to reports.
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Source: IAR, PAP, Reuters