The country, which does not recognise same-sex marriage, had argued that the American was not entitled to the EU residency rights awarded to spouses, the BBC reported.
But the European Court of Justice (ECJ) said the term "spouse" was gender neutral.
Adrian Coman and his American partner Clai Hamilton were married in Brussels in 2010.
The ECJ ruled that member states should recognise gay marriages contracted in fellow EU states, and grant couples the same residency rights that other families enjoy.
"Although the member states have the freedom whether or not to authorise marriage between persons of the same sex, they may not obstruct the freedom of residence of an EU citizen by refusing to grant his same-sex spouse, a national of a country that is not an EU Member State, a derived right of residence in their territory," the court said.
Following the ruling, Conan said: "We are grateful to the EU Court and to the many people and institutions who have supported us, and through us, other same-sex couples in a similar situation... It is human dignity that wins today."
EU law permits a non-EU spouse of an EU citizen to join his or her spouse in the member state where the European national resides, reports the BBC.
Same-sex marriage is legal in these EU countries: Belgium, Denmark, Finland, France, Germany, Ireland, Luxembourg, Netherlands, Portugal, Spain, Sweden, the UK (excluding Northern Ireland) and Malta.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
