The vote, condemned by numerous rights groups, is to be held on Oct 6-7.
Romania is to hold a referendum on Oct 6-7 on whether to amend the constitution's definition of marriage to a union between a man and a woman which would effectively block same-sex marriages.
The constitution currently states that marriage is between two "spouses" but makes no mention of gender.
The referendum — approved by the lower house of parliament and by the Senate after a 2016 petition from Coalitia pentru Familie collected three million signatures — aims to change the constitution so that it states that a union is strictly between "a man and a woman".
What happens if the definition amended?
If the referendum passes it would then ensure that same-sex marriages or civil unions, illegal in the country, remain so.
The referendum would also set the conservative eastern European country apart from most of the European Union where same-sex marriages or civil partnerships have gained recognition in the past few years.
The move to hold the referendum has been widely condemned by rights organisations with LGBT group MozaiQ denouncing it as "immoral." The group has called on Romanians to boycott the vote as the country only validates the result of a referendum if at least 30% of the 18-million electorate participates.
The vote also comes after Romania was ordered in June by the European Court of Justice to recognise the American partner of a Romanian national as his spouse so that he could legally reside in the country. The couple had married several years earlier in Belgium but wished to move to Romania.
Europe's top court ruled that, although a member state can decide whether to recognise sex-same unions, they must recognise unions performed in other EU countries and grant their spouses the same rights as any European citizen including "full freedom of movement, which is one of the four fundamentals of the European Union."