The Ipsos exit poll for broadcasters TVP, TVN and Polsat indicated Trzaskowski secured 50.3 percent of the vote, while his conservative rival, Karol Nawrocki, garnered 49.7 percent.
The polling company said its exit poll findings could vary by up to 2 percentage points from the final, official results.
The official results are expected to be announced on Monday.
If confirmed by the national electoral authority, Trzaskowski's win could bolster the pro-European government of Prime Minister Donald Tusk.
Trzaskowski, 53, a senior politician of Tusk's ruling Civic Coalition (KO), campaigned on a platform of strengthening Poland’s ties with the bloc and advancing judicial reforms.
Nawrocki, a conservative historian backed by the opposition Law and Justice (PiS) party, had pledged to veto liberal reforms, promote traditional Catholic values and align Poland more closely with Donald Trump's America.
The close race highlighted deep divisions in Polish society over the country’s direction, with the president holding significant veto power over legislation.
Trzaskowski’s win is expected to facilitate Tusk’s reform agenda, including efforts to reverse controversial judicial changes made by the previous government from 2015 to 2023.
The election saw high voter turnout, with 72.8 percent of eligible voters casting ballots, according to the exit poll.
In the first round of voting on May 18, no candidate won an outright majority, meaning a second round had to be held.
The Polish president is the head of state and has the power to veto legislation passed by parliament, a key prerogative in a country where traditionalists and liberals are bitterly divided.