The court said gay couples had the same rights to a private life and a family life as heterosexuals and thus should "benefit, in the long term, from legal... recognition of their rights and obligations".
The landmark ruling comes before a referendum planned for October 6 and 7, seeking to restrict the constitutional definition of "family" to heterosexual, married couples.
The vote was called by a group called "Coalition for the Family" and others close to the Orthodox Church.
The proposal is to change the constitution to stipulate that marriage is between a man and a woman, and not simply "spouses", as it currently states.
The plebiscite has already been criticised by Amnesty International, which says it would breach international human rights standards and amount to homophobic discrimination.
Thursday's ruling was in a case brought by a US-Romanian couple who had asked the authorities to recognise their marriage, registered in Belgium, so the American could move with his husband to Romania.
The constitutional court based its judgment on a ruling issued in June by the Court of Justice of the European Union.
Romania is a very conservative country where most people are members of the Orthodox Church, and only legalised homosexuality at the beginning of the 2000s.
Eyewitness? Submit your stories now via social or:
Email: eyewitness@pulse.ng
![The 10 warmest years have occurred since 2007 [Black Doctor]](data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D'http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg'%20width%3D'300'%20height%3D'172'%20data-ring-placeholder%3D'1'%3E%3C%2Fsvg%3E "The 10 warmest years have occurred since 2007 [Black Doctor]")
