Same-sex couples in Austria, a predominantly Roman Catholic nation of some 8.7 million people, have been allowed to enter civil partnerships since 2010. Under previous law, gay couples could join together in civil unions and were afforded all the legal rights of married heterosexual couples, but could not officially marry.
The ruling brings Austria up to speed with other European nations including Germany, Spain, France and the United Kingdom, all of which have legalized same-sex marriage in the past several years.
"Today, the differentiation between marriage and legally registered partnerships can no longer be upheld without discriminating against same-sex couples", the court said.
The European Court of Human Rights (ECtHR) has ruled that the European Convention on Human Rights (ECHR) requires member states to provide legal recognition, but does not require marriage to be opened to same-sex couples.
It said in a ruling published on Tuesday that restrictions on same-sex marriage will be lifted at the end of 2018, unless the government does so itself earlier.
Helmut Graupner, a lawyer for the two women who brought the case, wrote in a Facebook post that "today is a truly historic day". For example, since 2016 same-sex couples have been allowed to adopt children, and all couples have equal access to medically assisted reproduction.
The Constitutional Court said: 'The distinction between marriage and registered partnership cannot be maintained today without discriminating against same-sex couples'. Australia also voted in November to legalize same-sex marriage in a non-binding vote, which will likely become law. In other countries, including Chile and Taiwan, such legislation is being drafted. Men who have sex with men are also not allowed to donate blood.
According to The New York Times, the court advised that gay couples be allowed to marry starting in 2019, unless lawmakers act sooner.