The legislation would have allowed two adults regardless of gender, to sign a cohabitation agreement before a notary.
Such contracts would have regulated issues including housing rights, alimony, access to medical information, health insurance, caregiving leave and joint tax filings.
The bill, put forward by junior coalition partners PSL and the Left, had already cleared both houses of parliament – the first such proposal to do so in Poland's history.
Nawrocki said he could not accept a measure that risked eroding marriage's special status under Article 18 of the Polish constitution, which defines it as a union between a woman and a man.
He argued the bill would have created a new family-law institution mirroring marriage's core rights without its obligations, effectively introducing partnerships "through the back door."
Nawrocki added that the veto does not close the debate on addressing the practical problems faced by unmarried couples.
The president stressed he had signed nine other bills into law.
'A disgrace' – coalition reacts
The decision drew sharp criticism from governing coalition figures.
Defence Minister Władysław Kosiniak-Kamysz, who also serves as deputy PM, said the president had blocked positive change, choosing politics over making people's lives easier.
Deputy Prime Minister and Digital Affairs Minister Krzysztof Gawkowski called the veto "a disgrace," saying it told thousands of Poles their families and lives mattered less.
Education Minister Barbara Nowacka described it as a display of contempt, while Family and Social Policy Minister Agnieszka Dziemianowicz-Bąk said it stripped security from people in informal relationships.
(ał)
Source: PAP, onet.pl