The European Court of Human Rights on Tuesday condemned Poland for violating citizens' rights by failing to recognise and protect same-sex couples.
The verdict of the court, based in Strasbourg, follows complaints lodged by 10 Polish nationals living in same-sex relationships and whose application to get married was refused by the authorities.
Only a man and a woman could get married legally, the authorities told the applicants.
National courts rejected the legal challenges to the decision brought by the group, who then turned to the European court.
They argued that a lack of official recognition of their relationship brought them disadvantages, in terms of taxation, social rights and family law.
In its verdict, the ECHR backed their arguments, saying member states were required to provide a legal framework allowing same-sex couples to be granted adequate recognition and protection of their relationship.
This, the court said, did not have to take the form of traditional marriage, with the government free to determine the exact nature of the legal regime, so long as it was "adequate".
Marriage is currently the only legal way to formalise a relationship in Poland, the court noted.
The court said it had taken note of reports of increasingly hostile and homophobic attitudes towards sexual minorities displayed by high-ranking politicians from the ruling party in Poland.
It found that there was no basis for considering that affording legal recognition and protection to same-sex couples in a stable and committed relationship could in itself harm families constituted in the traditional way.
One of the plaintiffs, Krzysztof Alcer, said he was "very happy" about the court's decision.
"I hope that this would be the first strong impulse for change in favour of equality," he told AFP. "We expect the change to happen, we need this."
This year, the court had already found Russia, Romania, Bulgaria and Ukraine guilty of similar failings.
The ECHR is charged with ruling on violations of the European Convention of Human Rights, ratified by 46 countries.