The Liberian Senate on Friday voted unanimously to approve a constitutional amendment prohibiting marriage between gay couples.
While homosexuality is considered taboo in Liberia, and 'voluntary sodomy' considered a criminal offence, the question of gay marriage had not been expressly addressed in law.
The amendment was made to section 2.3 of the constitution, which bans marriage between people who are already wedded to others and between close family members.
It adds "or persons of the same sex" to the text, as read by the chairman of the judiciary committee Joseph Nagbe.
However, President Ellen Johnson Sirleaf has the final say and has previously said she would veto any bill on homosexuality, whether to legalise it or to toughen laws.
The bill was sponsored by Senator Jewell Taylor, ex-wife of former president Charles Taylor, who in May was sentenced to 50 years in prison for war crimes and crimes against humanity.
"My bill seeks to ensure that the fact that people of the same sex under our law should not be allowed to get married," Taylor said.
A second bill which seeks to make sexual relations between two people of the same sex a first-degree felony is currently before the House of Representatives.
The legislation comes after an acrimonious public debate on gay rights after a group of activists earlier this year began lobbying for a bill legalising same-sex marriage.
The leaders of the Movement in Defence of Gay and Lesbian Rights -- none of them gay themselves -- were mobbed and had to be rescued by police when they tried to campaign at a university campus.
This created a furore in the country and posed a thorny issue for Sirleaf, a Nobel Peace Prize winner who has approached the issue of gay rights uncomfortably.
She defended her country's stance in an interview with Britain's Guardian newspaper in March saying: "We like ourselves the way we are."
"We've got certain traditional values in our society that we would like to preserve," she says.
Liberia's main economic partner the United States said in December it would consider gay rights when handing out aid, which infuriated deeply traditional African countries, many of whom consider homosexuality "un-African".
US State Department spokeswoman Victoria Nuland said Tuesday: "I think if there were major pieces of legislation that discriminated against any group, we would have to take that into account in our relationship, and it would be a cause for concern."
In a letter to the Guardian, the Liberian government said there were no anti-gay laws "and as such the president could not be defending a law on homosexuality."
"What the president is on record as saying is that any law brought before her regarding homosexuality will be vetoed. This statement also applies to an initial attempt by two members of the Liberian legislature to introduce tougher laws targeting homosexuality," the letter said.
It added that the government believed current legislation was sufficient.
"The reality is that the status quo in Liberia has been one of tolerance and no one has ever been prosecuted under that law.
"The president also thinks that, with the unprecedented freedom of speech and expression Liberia enjoys today, our budding democracy will be strong enough to accommodate new ideas and debate both their value and Liberia's laws with openness, respect and independence."