Trzaskowski won 74.5 percent of the vote against his contender, Foreign Minister Radoslaw Sikorski, who garnered 25.25 percent in the secure-text vote of KO members.
The results of the primaries were announced by Donald Tusk, the prime minister and KO leader, on Saturday.
KO members cast 22,126 votes, Tusk said.
The ballot was carried out on Friday, with over 25,000 KO members eligible to vote.
KO is a coalition consisting of its backbone party, the centrist Civic Platform, and three smaller members.
Trzaskowski said after the results were announced that KO will soon show that yet again it is able to "wake up the whole of Poland."
In the 2020 presidential election, Trzaskowski lost by a small percentage of votes to incumbent President Andrzej Duda, an ally of the former ruling party, the socially-conservative Law and Justice (PiS). But there were concerns that the elections were not fair as the then-ruling PiS threw all the power of public media and state-owned companies behind Duda.
Trzaskowski said that in KO over 22,000 people had decided who would be the party's presidential candidate, but "in PiS one vote decides," as he referred to the fact that his key opponent will be likely picked by PiS leader Jaroslaw Kaczynski.
"In PiS, one vote matters, while we value every vote, and that's how we differ," he said.
"I'm convinced that we're coming out of these elections stronger, we're all stronger, and I have a very strong mandate and a lot of energy, determination and courage to win with PiS," he said.
In 2025, his main contender will be a PiS candidate who is yet to be named. (PAP)
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