Piątkowska, who represents the Civic Coalition in the Senate, unveiled her candidacy at a news conference in Warsaw attended by Tusk and Deputy Prime Minister Władysław Kosiniak-Kamysz, the leader of the PSL.
"I have decided to run for the office of mayor of Kraków," Piątkowska said, describing the southern city as the place where she studied, built her career and raised her son.
Tusk said Piątkowska's bid was jointly supported by the KO and PSL, calling her "an excellent candidate" for what he described as an important contest over the future of Poland's second-largest city.
Kosiniak-Kamysz also endorsed her candidacy, saying he believed she was well prepared to lead Kraków and promising the full support of his party.
The special election was triggered after voters removed Mayor Aleksander Miszalski from office in a referendum on May 24.
Piątkowska said she had heard residents' concerns about rising living costs, political divisions and the city's clean transport zone, which she said would require substantial revisions.
She pledged to focus on stability, professional management and cooperation with civic groups.
"Kraków does not need political conflict; it needs effective management," she said.
The date of the election has not yet been set. Under Polish electoral law, it should take place within 90 days of the official announcement of the referendum result.
However, Tusk said court challenges linked to the referendum could slightly delay the vote, with September the most likely timeframe.
Piątkowska was elected to the Senate in a 2025 by-election.
A lawyer by training, she has long been associated with Kraków. She served as director of the city's Strategy and Development Department from 2003 to 2012, helping shape municipal development plans and investment projects. She later worked in the economy ministry and at Poland's foreign investment agency.
Several other candidates have already declared their intention to run in the mayoral race, including contenders backed by the conservative Law and Justice (PiS) party, the far-right Confederation group and other political groupings.
Former Supreme Audit Office (NIK) chief Marian Banaś is also among the prospective contenders.
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Source: IAR, PAP