Robert Biedron, 42, rose to prominence as the country's first openly gay MP in 2011, elected as part of the 'Your Movement' party, a liberal, anti-clerical party.
In 2014, he became Mayor of the northern town of Slupsk.
On Sunday, he announced the name of his new party as Wiosna (Spring) and said he wanted to bring an end to deep divisions in Polish society.
In a bid to curb the influence of the Catholic Church, he said Wiosna would end tax breaks for the church, stop religious lessons in school, and guarantee access to contraception and the right to an
abortion until the 12th week of pregnancy.
The current governing Law and Justice party has close links to the Catholic Church and was recently in conflict with the EU over its appointment of judges.
Biedron told his audience at his party's launch in the capital, Warsaw:
"We want no more Polish-Polish war, we want mutual respect and dialogue.
"These last years have been cold and gloomy. Instead of talks we got unending conflict, instead of common good, party interests, instead of empathy, growing enmity. May this change at last."
Poland will hold both European and national parliamentary elections this year.
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