Slovenia joined a growing number of European countries that allow same-sex marriages late on Tuesday, almost three years after a similar law was rejected in a national referendum.
Parliament passed the law giving same-sex couples the right to marry and adopt children with 51 votes in favor and 28 against.
"This will be a step towards a truly patient and inclusive society with which Slovenia enters the twenty-first century," Matej Tasner Vatovec of center-left opposition party the United Left, which proposed the law, told parliament before the vote.
Two center-right opposition parties and several civic groups fiercely opposed the changes, particularly giving same-sex couples the right to adopt children.
The Civil Initiative for Family and Children's Rights, which opposes the changes, said on Wednesday it would push for a referendum on the law, which is similar to one rejected by Slovenians in a popular vote in 2012.
It is unlikely to succeed because Slovenia changed its referendum legislation in 2013 and no longer allows plebiscites on human rights issues.
A number of other European Union member countries have already recognized same-sex marriage, including Britain, France and Spain, although it remains a contentious issue in more socially conservative eastern states.
Croatia, which like Slovenia was part of former Yugoslavia, rejected same-sex marriage in a referendum in late 2013 and bans gay couples from adopting children, although they can register their partnerships.